ipv6: tcp: fix tcp_v6_conn_request()

Since linux 2.6.26 (commit c6aefafb

 : Add IPv6 support to TCP SYN
cookies), we can drop a SYN packet reusing a TIME_WAIT socket.

(As a matter of fact we fail to send the SYNACK answer)

As the client resends its SYN packet after a one second timeout, we
accept it, because first packet removed the TIME_WAIT socket before
being dropped.

This probably explains why nobody ever noticed or complained.

Reported-by: Jesse Young <jlyo@jlyo.org>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>