Problem:
 When osd is full, the client receives the notification
and cancels the ongoing writes. If the ongoing writes
are async, it could cause a dead lock as the async
callback registered also takes the 'client_lock' which
the handle_osd_map takes at the beginning.

 The op_cancel_writes calls the callback registered for
the async write synchronously holding the 'client_lock'
causing the deadlock.

Earlier approach:
  It was tried to solve this issue by calling 'op_cancel_writes'
without holding 'client_lock'. But this failed lock dependency
between objecter's 'rwlock' and async write's callback taking
'client_lock'. The 'client_lock' should always be taken before
taking 'rwlock'. So this approach is dropped against the current
approach.

Solution:
 Use C_OnFinisher for objecter async write callback i.e., wrap
the async write's callback using the Finisher. This queues the
callback to the Finisher's context queue which the finisher
thread picks up and executes thus avoiding the deadlock.

Testing:
 The fix is tested in the vstart cluster with the following reproducer.
 1. Mount the cephfs volume using nfs-ganesha at /mnt
 2. Run fio on /mnt on one terminal
 3. On the other terminal, blocklist the nfs client session
 4. The fio would hang

 It is reproducing in the vstart cluster most of the times. I think
that's because it's slow. The same test written for teuthology is
not reproducing the issue. The test expects one or more writes
to be on going in rados when the client is blocklisted for the deadlock
to be hit.

Stripped down version of Traceback:
----------
0  0x00007f4d77274960 in __lll_lock_wait ()
1  0x00007f4d7727aff2 in pthread_mutex_lock@@GLIBC_2.2.5 ()
2  0x00007f4d7491b0a1 in __gthread_mutex_lock (__mutex=0x7f4d200f99b0)
3  std::mutex::lock (this=<optimized out>)
4  std::scoped_lock<std::mutex>::scoped_lock (__m=..., this=<optimized out>, this=<optimized out>, __m=...)
5  Client::C_Lock_Client_Finisher::finish (this=0x7f4ca0103550, r=-28)
6  0x00007f4d74888dfd in Context::complete (this=0x7f4ca0103550, r=<optimized out>)
7  0x00007f4d7498850c in std::__do_visit<...>(...) (__visitor=...)
8  std::visit<Objecter::Op::complete(...) (__visitor=...)
9  Objecter::Op::complete(...) (e=..., e=..., r=-28, ec=..., f=...)
10 Objecter::Op::complete (e=..., r=-28, ec=..., this=0x7f4ca022c7f0)
11 Objecter::op_cancel (this=0x7f4d200fab20, s=<optimized out>, tid=<optimized out>, r=-28)
12 0x00007f4d7498ea12 in Objecter::op_cancel_writes (this=0x7f4d200fab20, r=-28, pool=103)
13 0x00007f4d748e1c8e in Client::_handle_full_flag (this=0x7f4d200f9830, pool=103)
14 0x00007f4d748ed20c in Client::handle_osd_map (m=..., this=0x7f4d200f9830)
15 Client::ms_dispatch2 (this=0x7f4d200f9830, m=...)
16 0x00007f4d75b8add2 in Messenger::ms_deliver_dispatch (m=..., this=0x7f4d200ed3e0)
17 DispatchQueue::entry (this=0x7f4d200ed6f0)
18 0x00007f4d75c27fa1 in DispatchQueue::DispatchThread::entry (this=<optimized out>)
19 0x00007f4d77277c02 in start_thread ()
20 0x00007f4d772fcc40 in clone3 ()
--------

Fixes: https://tracker.ceph.com/issues/68641
Signed-off-by: Kotresh HR <khiremat@redhat.com>
(cherry picked from commit 60c5801)