network/dhcp4: always find suitable gateway for destination address

network/dhcp4: always find suitable gateway for destination address

And if not found, refuse to configure the route.

If a DHCP server provides classless static or static routes, then we
should use the gateway for accessing a node in the range specified in
the route. E.g. if a DHCP server provides the default gateway is
192.168.0.1, and classless static route for 8.0.0.0/8 with gateway
192.168.0.2, then we should access 8.8.8.8 through 192.168.0.2 rather
than 192.168.0.1, but should use 192.168.0.1 for 9.9.9.9.

Fixes #28358.