zebra: Fix route replace v4 semantics with new system route

zebra: Fix route replace v4 semantics with new system route

When a new system route comes in and we have a pre-existing
non-system route we are not deleting the current system
route from the linux kernel.

Modify the code such that when a route replace is sent
to the kernel with a new route as a system route and
the old route as a non-system route do a delete of
the old route so it is no longer in the kernel.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>