network: ndisc - drop routes of lifetime 0

network: ndisc - drop routes of lifetime 0

A Lifetime of 0 indicates that the router is not a default router anymore
and associated default route should be discarded from host's routing table.

https://datatracker.ietf.org/doc/html/rfc4861
```
Router Lifetime
     16-bit unsigned integer.  The lifetime associated
     with the default router in units of seconds.  The
     field can contain values up to 65535 and receivers
     should handle any value, while the sending rules in
     Section 6 limit the lifetime to 9000 seconds.  A
     Lifetime of 0 indicates that the router is not a
     default router and SHOULD NOT appear on the default
     router list.  The Router Lifetime applies only to
     the router's usefulness as a default router; it
     does not apply to information contained in other
     message fields or options.  Options that need time
     limits for their information include their own
     lifetime fields.

```