Previously, we had Resource leak in Directory::for_each() where
directory streams opened with fdopendir() were never properly closed,
causing a 32KB leak per unclosed directory handle.

In this change, we add closedir() call to properly release directory stream
resources after enumeration completes.

ASan report:

```
Direct leak of 32816 byte(s) in 1 object(s) allocated from:
    #0 0x738387320e15 in malloc /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_malloc_linux.cpp:67
    #1 0x738381383514  (/usr/lib/libc.so.6+0xe3514) (BuildId: 468e3585c794491a48ea75fceb9e4d6b1464fc35)
    #2 0x738381383418 in fdopendir (/usr/lib/libc.so.6+0xe3418) (BuildId: 468e3585c794491a48ea75fceb9e4d6b1464fc35)
    #3 0x6433e1aefac4 in for_each<rgw::sal::Directory::fill_cache(const DoutPrefixProvider*, optional_yield, rgw::sal::fill_cache_cb_t&)::<lambda(char const*)> > /home/kefu/dev/ceph/src/rgw/driver/posix/rgw_sal_posix.cc:874
    #4 0x6433e1aa10ab in rgw::sal::Directory::fill_cache(DoutPrefixProvider const*, optional_yield, fu2::abi_310::detail::function<fu2::abi_310::detail::config<true, false, 16ul>, fu2::abi_310::detail::property<true, false, int (DoutPrefixProvider const*, rgw_bucket_dir_ent
ry&) const> > const&) /home/kefu/dev/ceph/src/rgw/driver/posix/rgw_sal_posix.cc:1077
    #5 0x6433e1aa8974 in rgw::sal::MPDirectory::fill_cache(DoutPrefixProvider const*, optional_yield, fu2::abi_310::detail::function<fu2::abi_310::detail::config<true, false, 16ul>, fu2::abi_310::detail::property<true, false, int (DoutPrefixProvider const*, rgw_bucket_dir_e
ntry&) const> > const&) /home/kefu/dev/ceph/src/rgw/driver/posix/rgw_sal_posix.cc:1384
    #6 0x6433e1abf1ee in rgw::sal::POSIXBucket::fill_cache(DoutPrefixProvider const*, optional_yield, fu2::abi_310::detail::function<fu2::abi_310::detail::config<true, false, 16ul>, fu2::abi_310::detail::property<true, false, int (DoutPrefixProvider const*, rgw_bucket_dir_e
ntry&) const> > const&) /home/kefu/dev/ceph/src/rgw/driver/posix/rgw_sal_posix.cc:2236
    #7 0x6433e1c43956 in file::listing::BucketCache<rgw::sal::POSIXDriver, rgw::sal::POSIXBucket>::fill(DoutPrefixProvider const*, file::listing::BucketCacheEntry<rgw::sal::POSIXDriver, rgw::sal::POSIXBucket>*, rgw::sal::POSIXBucket*, unsigned int, optional_yield) /home/kef
u/dev/ceph/src/rgw/driver/posix/bucket_cache.h:368
    #8 0x6433e1c1c71c in file::listing::BucketCache<rgw::sal::POSIXDriver, rgw::sal::POSIXBucket>::list_bucket(DoutPrefixProvider const*, optional_yield, rgw::sal::POSIXBucket*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, fu2::abi_310::d
etail::function<fu2::abi_310::detail::config<true, false, 16ul>, fu2::abi_310::detail::property<true, false, bool (rgw_bucket_dir_entry const&) const> >) /home/kefu/dev/ceph/src/rgw/driver/posix/bucket_cache.h:410
    #9 0x6433e1ac1829 in rgw::sal::POSIXBucket::list(DoutPrefixProvider const*, rgw::sal::Bucket::ListParams&, int, rgw::sal::Bucket::ListResults&, optional_yield) /home/kefu/dev/ceph/src/rgw/driver/posix/rgw_sal_posix.cc:2256
    #10 0x6433e1ae1302 in rgw::sal::POSIXMultipartUpload::list_parts(DoutPrefixProvider const*, ceph::common::CephContext*, int, int, int*, bool*, optional_yield, bool) /home/kefu/dev/ceph/src/rgw/driver/posix/rgw_sal_posix.cc:3692
    #11 0x6433e1ae2f3b in rgw::sal::POSIXMultipartUpload::complete(DoutPrefixProvider const*, optional_yield, ceph::common::CephContext*, std::map<int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<int>, std::allocator<std::pair<
int const, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >&, std::__cxx11::list<rgw_obj_index_key, std::allocator<rgw_obj_index_key> >&, unsigned long&, bool&, RGWCompressionInfo&, long&, std::__cxx11::basic_string<char, std::char_trait
s<char>, std::allocator<char> >&, ACLOwner&, unsigned long, rgw::sal::Object*, boost::container::flat_map<unsigned int, boost::container::flat_set<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::__cxx11::basic_string<char, std
::char_traits<char>, std::allocator<char> > >, void>, std::less<unsigned int>, void>&) /home/kefu/dev/ceph/src/rgw/driver/posix/rgw_sal_posix.cc:3770
    #12 0x6433e0fffb73 in POSIXMPObjectTest::create_MPObj(rgw::sal::MultipartUpload*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >) /home/kefu/dev/ceph/src/test/rgw/test_rgw_posix_driver.cc:1717
    ceph#13 0x6433e0e95ab9 in POSIXMPObjectTest_BucketList_Test::TestBody() /home/kefu/dev/ceph/src/test/rgw/test_rgw_posix_driver.cc:1863
    ceph#14 0x6433e1308d17 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/kefu/dev/ceph/src/googletest/googletest/src/gtest.cc:2653
```

Fixes https://tracker.ceph.com/issues/71505
Signed-off-by: Kefu Chai <tchaikov@gmail.com>