scan: wipe filters when dropping scanned data

scan: wipe filters when dropping scanned data

Fix clearing persistent filter state when clearing all
the state from a label_scan.

label_scan reads devs and saves info in bcache, lvmcache,
and in the persistent filter.  In some uncommon cases, an
lvm command wants to clear all info from a prior label_scan,
and repeat label_scan from scratch.  In these cases, info
in lvmcache, bcache and the persistent filter all need to
be cleared before repeating label_scan.

By missing the persistent filter wiping, outdated persistent
filter info, from a prior label_scan, could cause lvm to
incorrectly filter devices that change between polling intervals.
(i.e. if the device changes in such a way that the filtering
results change.)

A case where lvm wants to do multiple label_scans is a
polling command (like lvconvert --merge), when lvmpolld
has been disabled, so that the command itself needs to
to do repeated polling checks.