ipv6: When forwarding count rx stats on the orig netdev

Commit bdb7cc64 ("ipv6: Count interface receive statistics on the
ingress netdev") does not work when ip6_forward() executes on the skbs
with vrf-enslaved netdev. Use IP6CB(skb)->iif to get to the right one.

Add a selftest script to verify.

Fixes: bdb7cc64

 ("ipv6: Count interface receive statistics on the ingress netdev")
Signed-off-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20211014130845.410602-1-ssuryaextr@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>