zebra: dont update counter if outside of zebra ID range

zebra: dont update counter if outside of zebra ID range

When we receive a NHG from the kernel, we set the ID counter
to that to avoid using IDs owned from the kernel.

If we get one outside of zebra's range, lets not update it
since its probably one we created and never deleted anyway.

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