ipv6: tcp: send consistent flowlabel in TIME_WAIT state

After commit 1d13a96c ("ipv6: tcp: fix flowlabel value in ACK
messages"), we stored in tw_flowlabel the flowlabel, in the
case ACK packets needed to be sent on behalf of a TIME_WAIT socket.

We can use the same field so that RST packets sent from
TIME_WAIT state also use a consistent flowlabel.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Florent Fourcot <florent.fourcot@wifirst.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>