ipv6: Fix tcp_v6_send_response transport header setting.

My recent patch to remove the open-coded checksum sequence in
tcp_v6_send_response broke it as we did not set the transport
header pointer on the new packet.

Actually, there is code there trying to set the transport
header properly, but it sets it for the wrong skb ('skb'
instead of 'buff').

This bug was introduced by commit
a8fdf2b3 ("ipv6: Fix
tcp_v6_send_response(): it didn't set skb transport header")

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>