ipconfig: Do not filter out 0.0.0.0 gateway routes

This reverts commit cb05780d86c3 ("ipconfig: Don't add invalid gateway
routes") as it introduced regressions for IPv4LL setups.

0.0.0.0 is a valid gateway address for IPv4LL setups (see forwarding
rules in RFC3927).

Reported by Geoffrey Van Landeghem <geoffrey.vl@gmail.com>
1 file changed