netfilter: nft_fib: Fix for rpath check with VRF devices

[ Upstream commit 2a8a7c0eaa8747c16aa4a48d573aa920d5c00a5c ]

Analogous to commit b575b24b ("netfilter: Fix rpfilter
dropping vrf packets by mistake") but for nftables fib expression:
Add special treatment of VRF devices so that typical reverse path
filtering via 'fib saddr . iif oif' expression works as expected.

Fixes: f6d0cbcf ("netfilter: nf_tables: add fib expression")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>