tcp: add LINUX_MIB_TCPRETRANSFAIL counter

It might be useful to get a counter of failed tcp_retransmit_skb()
calls.

Reported-by: Satoru Moriya <satoru.moriya@hds.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 8c8de27..561550a 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2308,8 +2308,10 @@
 		if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
 			continue;
 
-		if (tcp_retransmit_skb(sk, skb))
+		if (tcp_retransmit_skb(sk, skb)) {
+			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL);
 			return;
+		}
 		NET_INC_STATS_BH(sock_net(sk), mib_idx);
 
 		if (inet_csk(sk)->icsk_ca_state == TCP_CA_Recovery)