[fix] Updated the way default interfaces are chosen in netlink.rs

[vats004]

This pr changes the way the default route and corresponding default interface are chosen

possible problem in issue 2117

1. default route is set by looking for any attrtibure that indicates the route being default route(by looking for Rta::Gateway)
2. and simply took the first matching route’s interface index and the corresponding interface as default

What this fix does :

1. Now, every default route(routes where rtm_dst_len == 0) is looked at
2. and its interface index and a “metric” value is looked at
3. whichever default route has the smallest metric, the interfave at its interface index is taken as the default interface

This should ensure that if multiple interfaces have “default routes,” the one with the lowest metric is chosen