libceph, ceph: move mdsmap.h to fs/ceph

The mdsmap.h is only used by CephFS, so move it to fs/ceph.

Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>