fix #3555: BTRFS: call DirPlugin's free_image correctly

fix #3555: BTRFS: call DirPlugin's free_image correctly

The method is only derived in the DirPlugin module from the base
Plugin, so we do not have it available there through a static module
method call using ::, but only when using a class dereference.

Other fix options would have been:

  PVE::Storage::Plugin::free_image(@_);

or:
  $class->SUPER::free_image($storeid, ...);

Signed-off-by: Hannes Laimer <h.laimer@proxmox.com>
[ Thomas: add some background to the commit message ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>