interface-ip: fix subnet route handling

interface-ip: fix subnet route handling

When the kernel subnet route has to be replaced, the cleanup call needs
to match the properties of the replacement route exactly, mainly the
metric and the routing table.
Fix handling this by embedding the device_route for the subnet in the
device_addr struct and using it in the cleanup path.

This fixes issues on config reload with changes to the routing table

Signed-off-by: Felix Fietkau <nbd@openwrt.org>