KEYS: Ceph: Use user_match()

Ceph can use user_match() instead of defining its own identical function.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>
cc: Tommi Virtanen <tommi.virtanen@dreamhost.com>