ceph: add a new vxattr to return auth mds for an inode

Add a new vxattr that shows what MDS is authoritative for an inode (if
we happen to have auth caps). If we don't have an auth cap for the inode
then just return -1.

URL: https://tracker.ceph.com/issues/1276


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