net: sock_free() optimizations

Avoid two extra instructions in sock_free(), to reload
skb->truesize and skb->sk

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -1339,9 +1339,10 @@ EXPORT_SYMBOL(sock_wfree);
 void sock_rfree(struct sk_buff *skb)
 {
 	struct sock *sk = skb->sk;
+	unsigned int len = skb->truesize;
 
-	atomic_sub(skb->truesize, &sk->sk_rmem_alloc);
-	sk_mem_uncharge(skb->sk, skb->truesize);
+	atomic_sub(len, &sk->sk_rmem_alloc);
+	sk_mem_uncharge(sk, len);
 }
 EXPORT_SYMBOL(sock_rfree);