ceph: allow ceph_put_mds_session to take NULL or ERR_PTR

...to simplify some error paths.

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