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/1276Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Luis Henriques <lhenriques@suse.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>