ceph: only choose one MDS who is in up:active state without laggy

Even the MDS is in up:active state, but it also maybe laggy. Here
will skip the laggy MDSs.

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