ceph: eliminate ceph_async_iput()

Now that we don't need to hold session->s_mutex or the snap_rwsem when
calling ceph_check_caps, we can eliminate ceph_async_iput and just use
normal iput calls.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>