pimd: Limit lookup of neighbor since we know we have one

pimd: Limit lookup of neighbor since we know we have one

When a new pim neighbor comes up, we need to go through and
mark nexthops that we have received from zebra for reachability
tracking so we can refigure stuff.  If we pass in the new neighbor
we can limit the search to those nexthops out the interface that
the new neighbor has come up.

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