ospf6d: Don't advertise AS-External LSAs into stub area

ospf6d: Don't advertise AS-External LSAs into stub area

If area is a normal area and has adjacencies up and then the user changes
the area to a stub area, the code was leaving existing AS-External LSAs in
the database and was sending AS-External LSAs into the stub area causing
the adjacency to stay in Ex-Start.   With this change we now cleanup the
AS-External LSAs that existed when area was not a stub and do not advertise
AS-External LSAs into the stub area.

Signed-off-by: Lynne Morrison <lynne@voltanet.io>