netfilter: nft_fib: allow from forward/input without iif selector

This removes the restriction of needing iif selector in the
forward/input hooks for fib lookups when requested result is
oif/oifname.

Removing this restriction allows "loose" lookups from the forward hooks.

Fixes: be8be04e

 ("netfilter: nft_fib: reverse path filter for policy-based routing on iif")
Signed-off-by: Eric Garver <eric@garver.life>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>