zebra: Make nexthop_active check use the same debug

zebra: Make nexthop_active check use the same debug

When debugging why a route was not successfully installed into the
rib, it would be preferable that the end user only have to turn
on `debug zebra rib detail` as that is what we have been telling
people to do for the last couple of years.  Consolidate *back*
to this.

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