[BRIDGE-NF]: Fix bridge-nf ipv6 length check

A typo caused some bridged IPv6 packets to get dropped randomly,
as reported by Sebastien Chaumontet. The patch below fixes this
(using skb->nh.raw instead of raw) and also makes the jumbo packet
length checking up-to-date with the code in
net/ipv6/exthdrs.c::ipv6_hop_jumbo.

Signed-off-by: Bart De Schuymer <bdschuym@pandora.be>
Signed-off-by: David S. Miller <davem@davemloft.net>