zebra: On if down/addr-del, process kernel routes

zebra: On if down/addr-del, process kernel routes

Since we don't have a daemon who's job is to handle kernel
routes and we don't get an explicit route delete anymore if
nexthops become unreachable from the kernel, zebra must
re-process kernel routes itself to make sure they are still valid.

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