userns: Add an explicit reference to the parent user namespace

I am about to remove the struct user_namespace reference from struct user_struct.
So keep an explicit track of the parent user namespace.

Take advantage of this new reference and replace instances of user_ns->creator->user_ns
with user_ns->parent.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

security/commoncap.c

@@ -92,7 +92,7 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 		 *If you have a capability in a parent user ns, then you have
 		 * it over all children user namespaces as well.
 		 */
-		targ_ns = targ_ns->creator->user_ns;
+		targ_ns = targ_ns->parent;
 	}
 
 	/* We never get here */