udp6: add a missing call into udp_fail_queue_rcv_skb tracepoint

The tracepoint has existed for 12 years, but it only covered udp
over the legacy IPv4 protocol. Having it enabled for udp6 removes
the unnecessary difference in error visibility.

Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
Fixes: 296f7ea7 ("udp: add tracepoints for queueing skb to rcvbuf")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>