netfilter: fib: check correct rtable in vrf setups

[ Upstream commit 05ef7055 ]

We need to init l3mdev unconditionally, else main routing table is searched
and incorrect result is returned unless strict (iif keyword) matching is
requested.

Next patch adds a selftest for this.

Fixes: 2a8a7c0e ("netfilter: nft_fib: Fix for rpath check with VRF devices")
Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1761


Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>