AF_UNIX: Implement SO_TIMESTAMP and SO_TIMETAMPNS on Unix sockets

Userspace applications can already request to receive timestamps with:
setsockopt(sockfd, SOL_SOCKET, SO_TIMESTAMP, ...)

Although setsockopt() returns zero (success), timestamps are not added to the
ancillary data. This patch fixes that on SOCK_DGRAM and SOCK_SEQPACKET Unix
sockets.

Signed-off-by: Alban Crequy <alban.crequy@collabora.co.uk>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -1511,6 +1511,8 @@ restart:
 		goto restart;
 	}
 
+	if (sock_flag(other, SOCK_RCVTSTAMP))
+		__net_timestamp(skb);
 	skb_queue_tail(&other->sk_receive_queue, skb);
 	unix_state_unlock(other);
 	other->sk_data_ready(other, len);
@@ -1722,6 +1724,9 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
 	if (err)
 		goto out_free;
 
+	if (sock_flag(sk, SOCK_RCVTSTAMP))
+		__sock_recv_timestamp(msg, sk, skb);
+
 	if (!siocb->scm) {
 		siocb->scm = &tmp_scm;
 		memset(&tmp_scm, 0, sizeof(tmp_scm));