netlink: Fix RTA_MULTIPATH on RTM_GETROUTE

[pouria]

Summary

Before this patch, netlink never returned RTA_MULTIPATH.
Also, add RTA_MULTIPATH attribute in way that don't confuse
non-mpath support consumers.

[pouria]

sys/netlink/route/rt.c

189

For reviewers, Without this, non-mpath aware apps can't find their gw, therefore, we choose one here for netlink users. see line 658 below.

349–353

For reviewers, without these changes here + removal of nhop_select_func() from line 658 it was impossible to return RTA_MULTIPATH.
Since we overwrite nhgrp of rnd by nhop_select_func() to nhop. So this patch will restore the intended behavior.

[pouria]

This comment was removed by pouria.

[pouria]

Also return the weight of selected nhop from nhgrp.