[IPV6]: Mischecked tw match in __inet6_check_established.

When looking for a conflicting connection the !sk->sk_bound_dev_if
check is performed only for live sockets, but not for timewait-ed.

This is not the case for ipv4, for __inet6_lookup_established in
both ipv4 and ipv6 and for other places that check for tw-s.

Was this missed accidentally? If so, then this patch fixes it and
besides makes use if the dif variable declared in the function.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>