tcp: add tcp_wstamp_ns socket field

TCP will soon provide earliest departure time on TX skbs.
It needs to track this in a new variable.

tcp_mstamp_refresh() needs to update this variable, and
became too big to stay an inline.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -248,6 +248,8 @@ struct tcp_sock {
 		syn_smc:1;	/* SYN includes SMC */
 	u32	tlp_high_seq;	/* snd_nxt at the time of TLP retransmit. */
 
+	u64	tcp_wstamp_ns;	/* departure time for next sent data packet */
+
 /* RTT measurement */
 	u64	tcp_mstamp;	/* most recent packet received/sent */
 	u32	srtt_us;	/* smoothed round trip time << 3 in usecs */