np->cork.tclass is used only in cork'ed context.
Otherwise, np->tclass should be used.

Bug#7096 reported by Remi Denis-Courmont <rdenis@simphalempin.com>.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>