ifconfig_netlink.c: optimise non-listing case

This change produced by melifaro and the commit is concerted with him
as he is pretty busy IRL these days.

The change restores performance of /sbin/ifconfig utility
for non-listing case after it switched from rtsock to netlink(4) API
in FreeBSD 14+.

PR: 287872
MFC-after: 2 weeks