udp: no longer touch sk->sk_refcnt in early demux

After commits ca065d0c ("udp: no longer use SLAB_DESTROY_BY_RCU")
and 7ae215d2 ("bpf: Don't refcount LISTEN sockets in sk_assign()")
UDP early demux no longer need to grab a refcount on the UDP socket.

This save two atomic operations per incoming packet for connected
sockets.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Joe Stringer <joe@wand.net.nz>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -2568,11 +2568,12 @@ int udp_v4_early_demux(struct sk_buff *skb)
 					     uh->source, iph->saddr, dif, sdif);
 	}
 
-	if (!sk || !refcount_inc_not_zero(&sk->sk_refcnt))
+	if (!sk)
 		return 0;
 
 	skb->sk = sk;
-	skb->destructor = sock_efree;
+	DEBUG_NET_WARN_ON_ONCE(sk_is_refcounted(sk));
+	skb->destructor = sock_pfree;
 	dst = rcu_dereference(sk->sk_rx_dst);
 
 	if (dst)

net/ipv6/udp.c

@@ -1098,11 +1098,12 @@ void udp_v6_early_demux(struct sk_buff *skb)
 	else
 		return;
 
-	if (!sk || !refcount_inc_not_zero(&sk->sk_refcnt))
+	if (!sk)
 		return;
 
 	skb->sk = sk;
-	skb->destructor = sock_efree;
+	DEBUG_NET_WARN_ON_ONCE(sk_is_refcounted(sk));
+	skb->destructor = sock_pfree;
 	dst = rcu_dereference(sk->sk_rx_dst);
 
 	if (dst)