tcp: do not use ipv6 header for ipv4 flow

When a dual stack tcp listener accepts an ipv4 flow,
it should not attempt to use an ipv6 header or tcp_v6_iif() helper.

Fixes: 1397ed35 ("ipv6: add flowinfo for tcp6 pkt_options for all cases")
Fixes: df3687ff ("ipv6: add the IPV6_FL_F_REFLECT flag to IPV6_FL_A_GET")
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>