zebra: disable dependent backpointers for backup nexthops

zebra: disable dependent backpointers for backup nexthops

Because the backup nexthop groups currently are more like pseudo-NHEs
(they don't have IDs and are not inserted into the ID table or
hashed), they can't really have this depends/dependents relationship
yet in both directions. Some work needs to be done there to make
them more like first class citizens like "normal" NHGs to enable
this.

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