zebra: Allow multiple rib deletes from linux kernel

zebra: Allow multiple rib deletes from linux kernel

The kernel can send a DELROUTE with a individual
nexthop.  Technically this is meant to delete that
individual nexthop from the route but zebra
has no way to do this currently.  So we just delete
the route.

V4 -> Never sends a DELROUTE with multiple nexthops
as a way to modify the rib.  It sends a a NEWROUTE
with RTM_REPLACE with the new appropriate route.

V6 -> Sends a DELROUTE with multiple nexthops
which is supposed to be interpreted as a
subtraction from the route.

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