zebra: fix refcnt/rib issues in NHG replace/delete

zebra: fix refcnt/rib issues in NHG replace/delete

Fix some reference counting issues seen when replacing
a NHG and deleting one.

For replacement, we should end with the same refcnt on the new
one.

For delete, its the caller's job to decrement its ref after
its done with it.

Further, update routes in the rib with the new pointer after replace.

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