ceph: rename get_session and switch to use ceph_get_mds_session

Just in case the session's refcount reach 0 and is releasing, and
if we get the session without checking it, we may encounter kernel
crash.

Rename get_session to ceph_get_mds_session and make it global.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>