ipv6: Fix tcp_v6_send_response(): it didn't set skb transport header

Here is a patch which fixes an issue observed when using TCP over IPv6
and AH from IPsec.

When a connection gets closed the 4-way method and the last ACK from
the server gets dropped, the subsequent FINs from the client do not
get ACKed because tcp_v6_send_response does not set the transport
header pointer. This causes ah6_output to try to allocate a lot of
memory, which typically fails, so the ACKs never make it out of the
stack.

I have reproduced the problem on kernel 2.6.7, but after looking at
the latest kernel it seems the problem is still there.

Signed-off-by: Cosmin Ratiu <cratiu@ixiacom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>