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>