ospf6d: withdraw redistributed prefixes based on prefix-list changes

ospf6d: withdraw redistributed prefixes based on prefix-list changes

Problem reported that when an a previously advertised redistributed
route should be withdrawn based on a prefix-list change or route-map
deletion, the external LSAs would remain in the database and not be
withdrawn from peers. This fix does the withdraw when the prefix-list
is changed or route-map is deleted.

Ticket: CM-28944
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>