tcp: factorize sk_txhash init

Neal suggested to move sk_txhash init into tcp_create_openreq_child(),
called both from IPv4 and IPv6.

This opportunity was missed in commit 58d607d3 ("tcp: provide
skb->hash to synack packets")

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>