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/include/linux/snmp.h b/include/linux/snmp.h
index c1241c42..8ee8af4e6 100644
--- a/include/linux/snmp.h
+++ b/include/linux/snmp.h
@@ -232,6 +232,7 @@
 	LINUX_MIB_TCPTIMEWAITOVERFLOW,		/* TCPTimeWaitOverflow */
 	LINUX_MIB_TCPREQQFULLDOCOOKIES,		/* TCPReqQFullDoCookies */
 	LINUX_MIB_TCPREQQFULLDROP,		/* TCPReqQFullDrop */
+	LINUX_MIB_TCPRETRANSFAIL,		/* TCPRetransFail */
 	__LINUX_MIB_MAX
 };