iproute: fix printing resolved localhost

iproute: fix printing resolved localhost

format_host_rta_r might return a cached hostname
via its return value and not use the input buffer.

Before:

$ ip -resolve -6 route
 dev lo proto kernel metric 256 pref medium

After:

$ ip/ip -resolve -6 route
localhost dev lo proto kernel metric 256 pref medium

Bug-Debian: https://bugs.debian.org/983591

Reported-by: Axel Scheepers <axel.scheepers76@gmail.com>
Signed-off-by: Luca Boccassi <bluca@debian.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>