postinst: set custom LVM settings

postinst: set custom LVM settings

now that we no longer ship our own LVM packages, set the relevant
filtering options here if they are missing.

for an upgrade from PVE 6.x, the following two scenarios are likely:

A: user edited config provided by our old lvm2 package. it likely
contains our (or a modified) global_filter, but the old scan_lvs
default. in this case we ignore global_filter as long as it contains our
'don't scan zvols' entry, and set scan_lvs to false.

B: config provided by our old lvm2 package was taken over by default
config from stock lvm2 package. scan_lvs defaults to false already, but
global_filter is unset (scan everything), so we need to set our own
global_filter excluding zvols.

other combinations should be handled fine as well.

for new installs (installer, install on top of Debian Bullseye) we are
always in scenario B.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>