tcp: md5: add LINUX_MIB_TCPMD5FAILURE counter

Adds SNMP counter for drops caused by MD5 mismatches.

The current syslog might help, but a counter is more precise and helps
monitoring.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 9f665b6..1ed015e 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -257,6 +257,7 @@
 	SNMP_MIB_ITEM("TCPSpuriousRTOs", LINUX_MIB_TCPSPURIOUSRTOS),
 	SNMP_MIB_ITEM("TCPMD5NotFound", LINUX_MIB_TCPMD5NOTFOUND),
 	SNMP_MIB_ITEM("TCPMD5Unexpected", LINUX_MIB_TCPMD5UNEXPECTED),
+	SNMP_MIB_ITEM("TCPMD5Failure", LINUX_MIB_TCPMD5FAILURE),
 	SNMP_MIB_ITEM("TCPSackShifted", LINUX_MIB_SACKSHIFTED),
 	SNMP_MIB_ITEM("TCPSackMerged", LINUX_MIB_SACKMERGED),
 	SNMP_MIB_ITEM("TCPSackShiftFallback", LINUX_MIB_SACKSHIFTFALLBACK),