udp/v6: prefetch rmem_alloc in udp6_queue_rcv_skb()

very similar to commit dd99e425be ("udp: prefetch
rmem_alloc in udp_queue_rcv_skb()"), this allows saving a cache
miss when the BH is bottle-neck for UDP over ipv6 packet
processing, e.g. for small packets when a single RX NIC ingress
queue is in use.

Performances under flood when multiple NIC RX queues used are
unaffected, but when a single NIC rx queue is in use, this
gives ~8% performance improvement.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/udp.c

@@ -630,6 +630,7 @@ static int udpv6_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
 		}
 	}
 
+	prefetch(&sk->sk_rmem_alloc);
 	if (rcu_access_pointer(sk->sk_filter) &&
 	    udp_lib_checksum_complete(skb))
 		goto csum_error;