ospfd: blackhole route removal for area range

ospfd: blackhole route removal for area range

ISSUE

When an area range is created in which there the sub-area has routes that are
smaller than the range, an ABR creates a blackhole route to cover the range.
When the range is removed, the blackhole route is not removed.

--A----B----C---

B is an ABR with A in area 1 and C in area 0.  If A advertises `10.2.0.0/30` and
`10.2.0.4/30` and B is configured with `area 0.0.0.1 range 10.2.0.0/29` a
blackhole is created on B (`blackhole 10.2.0.0/29 proto zebra`).  When the
area/range is removed via the command line, the blackhole remains in existence
even though the "range" route is removed from area 0 and the individual routes
are propagated.

PATCH

The reason for this behavior is that, prior to this patch, the range is deleted
from the area's list, so when ospf_abr_manage_discard_routes() gets called,
there is nothing to clean up.  The patch removes the discard route as part of
the processing of the command line (ospf_area_range_unset()).

Signed-off-by: JR Rivers <jrrivers@cumulusnetworks.com>
Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Reviewed-by: Shrijeet Mukherjee <shm@cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>