ipv6: ip6_forward: perform skb->pkt_type check at the beginning

Packets which have L2 address different from ours should be
already filtered before entering into ip6_forward().

Perform that check at the beginning to avoid processing such packets.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>