tcp: use MAX_TCP_HEADER in tcp_stream_alloc_skb

Both IPv4 and IPv6 uses same reserve, no need risking
cache line misses to fetch its value.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -867,7 +867,7 @@ struct sk_buff *tcp_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp,
 	if (unlikely(tcp_under_memory_pressure(sk)))
 		sk_mem_reclaim_partial(sk);
 
-	skb = alloc_skb_fclone(size + sk->sk_prot->max_header, gfp);
+	skb = alloc_skb_fclone(size + MAX_TCP_HEADER, gfp);
 	if (likely(skb)) {
 		bool mem_scheduled;
 
@@ -878,7 +878,7 @@ struct sk_buff *tcp_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp,
 			mem_scheduled = sk_wmem_schedule(sk, skb->truesize);
 		}
 		if (likely(mem_scheduled)) {
-			skb_reserve(skb, sk->sk_prot->max_header);
+			skb_reserve(skb, MAX_TCP_HEADER);
 			/*
 			 * Make sure that we have exactly size bytes
 			 * available to the caller, no more, no less.