ipv6: udp: fix UDP_MIB_IGNOREDMULTI updates

IPv6 counters updates use a different macro than IPv4.

Fixes: 36cbb245 ("udp: Increment UDP_MIB_IGNOREDMULTI for arriving unmatched multicasts")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Rick Jones <rick.jones2@hp.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>