ipv6: tcp: fix endianness annotation in tcp_v6_send_response

Cc: Florent Fourcot <florent.fourcot@enst-bretagne.fr>
Fixes: 1d13a96c ("ipv6: tcp: fix flowlabel value in ACK messages send from TIME_WAIT")
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>