client: adjust `Fb` cap ref count check during synchronous fsync()

client: adjust `Fb` cap ref count check during synchronous fsync()

cephfs client holds a ref on Fb caps when handing out a write delegation[0].
As fsync from (Ganesha) client holding write delegation will block indefinitely[1]
waiting for cap ref for Fb to drop to 0, which will never happen until the
delegation is returned/recalled.

[0]: https://github.com/ceph/ceph/blob/main/src/client/Delegation.cc#L71
[1]: https://github.com/ceph/ceph/blob/main/src/client/Client.cc#L12438

If an inode has been write delegated, adjust for cap reference count
check in fsync().

Note: This only workls for synchronous fsync() since `client_lock` is
held for the entire duration of the call (at least till the patch leading
upto the reference count check). Asynchronous fsync() needs to be fixed
separately (as that can drop `client_lock`).

Fixes: https://tracker.ceph.com/issues/73298
Signed-off-by: Venky Shankar <vshankar@redhat.com>