netfilter: bridge: Don't sabotage nf_hook calls for an l3mdev slave

Followup to a173f066 ("netfilter: bridge: Don't sabotage nf_hook
calls from an l3mdev"). Some packets (e.g., ndisc) do not have the skb
device flipped to the l3mdev (e.g., VRF) device. Update ip_sabotage_in
to not drop packets for slave devices too. Currently, neighbor
solicitation packets for 'dev -> bridge (addr) -> vrf' setups are getting
dropped. This patch enables IPv6 communications for bridges with an
address that are enslaved to a VRF.

Fixes: 73e20b76

 ("net: vrf: Add support for PREROUTING rules on vrf device")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>