libceph: allow ceph_buffer_put() to receive a NULL ceph_buffer

[ Upstream commit 5c498950f730aa17c5f8a2cdcb903524e4002ed2 ]

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>