zebra: Check active on OLD system/kernel routes

zebra: Check active on OLD system/kernel routes

We can assume that system/kernel routes are valid indeed
if this is our first time procesing them. But since we don't
get explicit deletion events for kernel routes anymore, we
have to be prepared to process them if the nexthop becomes
unreachable for instance. Therefore, if the route is not NEW,
then don't assume its valid.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>