net: vrf: determine the dst using the original ifindex for multicast

Multicast packets received on an interface bound to a VRF are marked as
belonging to the VRF and the skb device is updated to point to the VRF
device itself. This was fine even when a route was associated to a
device as when performing a fib table lookup 'oif' in fib6_table_lookup
(coming from 'skb->dev->ifindex' in ip6_route_input) was set to 0 when
FLOWI_FLAG_SKIP_NH_OIF was set.

With commit 40867d74 ("net: Add l3mdev index to flow struct and
avoid oif reset for port devices") this is not longer true and multicast
traffic is not received on the original interface.

Instead of adding back a similar check in fib6_table_lookup determine
the dst using the original ifindex for multicast VRF traffic. To make
things consistent across the function do the above for all strict
packets, which was the logic before commit 6f12fa77 ("vrf: mark skb
for multicast or link-local as enslaved to VRF"). Note that reverting to
this behavior should be fine as the change was about marking packets
belonging to the VRF, not about their dst.

Fixes: 40867d74

 ("net: Add l3mdev index to flow struct and avoid oif reset for port devices")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20221220171825.1172237-1-atenart@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>