udp: Increment UDP_MIB_IGNOREDMULTI for arriving unmatched multicasts

As NIC multicast filtering isn't perfect, and some platforms are
quite content to spew broadcasts, we should not trigger an event
for skb:kfree_skb when we do not have a match for such an incoming
datagram.  We do though want to avoid sweeping the matter under the
rug entirely, so increment a suitable statistic.

This incorporates feedback from David L. Stevens, Karl Neiss and Eric
Dumazet.

V3 - use bool per David Miller

Signed-off-by: Rick Jones <rick.jones2@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index f0d4eb8..6513ade 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -181,6 +181,7 @@
 	SNMP_MIB_ITEM("RcvbufErrors", UDP_MIB_RCVBUFERRORS),
 	SNMP_MIB_ITEM("SndbufErrors", UDP_MIB_SNDBUFERRORS),
 	SNMP_MIB_ITEM("InCsumErrors", UDP_MIB_CSUMERRORS),
+	SNMP_MIB_ITEM("IgnoredMulti", UDP_MIB_IGNOREDMULTI),
 	SNMP_MIB_SENTINEL
 };