Signed-off-by: Venky Shankar <vshankar@redhat.com>

To allow the client to hold Fb caps for an extended period of
time, to allow an asynchronous fsync to intervene and block, so
as to hunt [0].

[0]: https://tracker.ceph.com/issues/71510

Signed-off-by: Venky Shankar <vshankar@redhat.com>

With the prior commit that introduces a synthetic delay in write
operation so as to write a test reproducer which would interleave
asynchronous fsync and an operation that makes the MDS send a early
reply to the client (therefore, having the client track the early
replied response for an inode in Inode::unsafe_ops). Now, this is
enough to trick the client into the code path that causes a buggy
reference drop for the request (MetaRequest), but, hitting the
_exact_ crash backtrace requires the request to be a in various
[x]list's.

This last bit is tricky to synthetically massage in the test. So,
in order to catch the buggy reference drop, it would suffice to
assert on the reference count dropping to less than zero (0).

Signed-off-by: Venky Shankar <vshankar@redhat.com>

…rrently

This synthetic reproducer does three things:

- setup a client mount with a configuration to delay write operations and
  initiates a write operation via a thread.
- a thread that invokes asynchronous fsync
- a thread that invokes setxattr for the client to track early replies

Without the fix[0], the test reproduces the following crash:

```
/home/vshankar/ceph/src/client/Client.cc: In function 'void Client::put_request(MetaRequest*)' thread 7f7210ff9640 time 2025-06-03T09:34:45.634974+0000
/home/vshankar/ceph/src/client/Client.cc: 2290: FAILED ceph_assert(request->ref >= 1)
 ceph version 20.3.0-673-gdd152807f7e (dd15280) tentacle (dev - Debug)
 1: (ceph::__ceph_assert_fail(char const*, char const*, int, char const*)+0x119) [0x7f72222ebb98]
 2: (ceph::__ceph_assert_fail(ceph::assert_data const&)+0x17) [0x7f72222ebedc]
 3: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0x6a075) [0x7f7222e6a075]
 4: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0xb8289) [0x7f7222eb8289]
 5: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0xee951) [0x7f7222eee951]
 6: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0xf167c) [0x7f7222ef167c]
 7: (Context::complete(int)+0x9) [0x7f7222e5949d]
 8: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0x16a853) [0x7f7222f6a853]
 9: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0xa7cc5) [0x7f7222ea7cc5]
 10: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0xf128d) [0x7f7222ef128d]
 11: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0x16e09d) [0x7f7222f6e09d]
 12: (Context::complete(int)+0x9) [0x7f7222e5949d]
 13: /home/vshankar/ceph/build/lib/libcephfs.so.2(+0x6d108) [0x7f7222e6d108]
 14: (Context::complete(int)+0x9) [0x7f7222e5949d]
 15: (Finisher::finisher_thread_entry()+0x665) [0x7f722226fdc1]
 16: (Finisher::FinisherThread::entry()+0xd) [0x7f7222270ddf]
 17: (Thread::entry_wrapper()+0x2f) [0x7f72222b88f5]
 18: (Thread::_entry_func(void*)+0x9) [0x7f72222b8907]
 19: /lib64/libc.so.6(+0x89e92) [0x7f7221089e92]
 20: /lib64/libc.so.6(+0x10ef20) [0x7f722110ef20]
[1]    2162689 IOT instruction (core dumped)  ./bin/ceph_test_libcephfs --gtest_filter=LibCephFS.ConcurrentWriteAndFsync
```

[0]: ceph#63619

Fixes: http://tracker.ceph.com/issues/71515
Signed-off-by: Venky Shankar <vshankar@redhat.com>