tcp: fix under-accounting retransmit SNMP counters

This patch fixes these under-accounting SNMP rtx stats
LINUX_MIB_TCPFORWARDRETRANS
LINUX_MIB_TCPFASTRETRANS
LINUX_MIB_TCPSLOWSTARTRETRANS
when retransmitting TSO packets

Fixes: 10d3be569243 ("tcp-tso: do not split TSO packets at retransmit time")
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -2831,7 +2831,7 @@ begin_fwd:
 		if (tcp_retransmit_skb(sk, skb, segs))
 			return;
 
-		NET_INC_STATS(sock_net(sk), mib_idx);
+		NET_ADD_STATS(sock_net(sk), mib_idx, tcp_skb_pcount(skb));
 
 		if (tcp_in_cwnd_reduction(sk))
 			tp->prr_out += tcp_skb_pcount(skb);