ospfd: When a neighbor goes down clear the oi->obuf if we can

ospfd: When a neighbor goes down clear the oi->obuf if we can

When a neighbor goes down on an interface and that interface
has no more neighbors in a viable state where packets should
be being sent, then let's clear up the oi->obuf associated
with the interface the neighbor is on.

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