userns: Convert audit to work with user namespaces enabled

- Explicitly format uids gids in audit messges in the initial user
  namespace. This is safe because auditd is restrected to be in
  the initial user namespace.

- Convert audit_sig_uid into a kuid_t.

- Enable building the audit code and user namespaces at the same time.

The net result is that the audit subsystem now uses kuid_t and kgid_t whenever
possible making it almost impossible to confuse a raw uid_t with a kuid_t
preventing bugs.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

kernel/audit.c

@@ -105,7 +105,7 @@ static int	audit_backlog_wait_time = 60 * HZ;
 static int	audit_backlog_wait_overflow = 0;
 
 /* The identity of the user shutting down the audit system. */
-uid_t		audit_sig_uid = -1;
+kuid_t		audit_sig_uid = INVALID_UID;
 pid_t		audit_sig_pid = -1;
 u32		audit_sig_sid = 0;
 
@@ -853,7 +853,7 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 				security_release_secctx(ctx, len);
 			return -ENOMEM;
 		}
-		sig_data->uid = audit_sig_uid;
+		sig_data->uid = from_kuid(&init_user_ns, audit_sig_uid);
 		sig_data->pid = audit_sig_pid;
 		if (audit_sig_sid) {
 			memcpy(sig_data->ctx, ctx, len);