tcp: add SRTT to SCM_TIMESTAMPING_OPT_STATS

Add TCP_NLA_SRTT to SCM_TIMESTAMPING_OPT_STATS that reports the smoothed
round trip time in microseconds (tcp_sock.srtt_us >> 3).

Signed-off-by: Yousuk Seung <ysseung@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tcp.h

@@ -266,6 +266,7 @@ enum {
 	TCP_NLA_BYTES_RETRANS,	/* Data bytes retransmitted */
 	TCP_NLA_DSACK_DUPS,	/* DSACK blocks received */
 	TCP_NLA_REORD_SEEN,	/* reordering events seen */
+	TCP_NLA_SRTT,		/* smoothed RTT in usecs */
 };
 
 /* for TCP_MD5SIG socket option */

net/ipv4/tcp.c

@@ -3242,6 +3242,7 @@ static size_t tcp_opt_stats_get_size(void)
 		nla_total_size_64bit(sizeof(u64)) + /* TCP_NLA_BYTES_RETRANS */
 		nla_total_size(sizeof(u32)) + /* TCP_NLA_DSACK_DUPS */
 		nla_total_size(sizeof(u32)) + /* TCP_NLA_REORD_SEEN */
+		nla_total_size(sizeof(u32)) + /* TCP_NLA_SRTT */
 		0;
 }
 
@@ -3295,6 +3296,7 @@ struct sk_buff *tcp_get_timestamping_opt_stats(const struct sock *sk)
 			  TCP_NLA_PAD);
 	nla_put_u32(stats, TCP_NLA_DSACK_DUPS, tp->dsack_dups);
 	nla_put_u32(stats, TCP_NLA_REORD_SEEN, tp->reord_seen);
+	nla_put_u32(stats, TCP_NLA_SRTT, tp->srtt_us >> 3);
 
 	return stats;
 }