ceph: send LSSNAP request to auth mds of directory inode

Snapdir inode has no capability. __choose_mds() should choose mds
base on capabilities of snapdir's parent inode.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>