ceph: add ceph.caps vxattr

Add a new vxattr that allows userspace to list the caps for a specific
directory or file.

[ jlayton: change format delimiter to '/' ]

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