ceph: properly put ceph_string reference after async create attempt

The reference acquired by try_prep_async_create is currently leaked.
Ensure we put it.

Cc: stable@vger.kernel.org
Fixes: 9a8d03ca ("ceph: attempt to do async create when possible")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>