zebra: simplify and optimize vrf display in show ip route

zebra: simplify and optimize vrf display in show ip route

In all outputs (text and json): simplify and optimize the vrf name
display, use the vrf_id_to_name() handler.

Note: vrf_id_to_name() has a safeguard system that prevents from
crashing when the vrf cannot be found because it changed in some
(unexpected) manner, it returns "n/a".

Note: "vrf n/a" will now be displayed instead of "vrf UNKNOWN" in this
case, like in most other frr components.

This safeguard was missing for show ip route json, so this
optimization also fixes a potential crash.

Signed-off-by: Christophe Gouault <christophe.gouault@6wind.com>