zebra/vty: use prefix2str and unify show ip/ipv6 route code

Use prefix2str where possible. As now ip/ipv6 are practically
identical, they are merged removing unneeded code duplication.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
1 file changed