btrfs: call free_image correctly

btrfs: call free_image correctly

Currently, 'PVE::Storage::DirPlugin' is implicitly passed along as
$class, which means that if the base class's free_image calls another
method (e.g.  filesystem_path) then the DirPlugin's method will be
used, rather than the one from BTRFSPlugin. Change it so that $class
itself is passed along.

See also commit 279d9de5108f6fc6f2d31f77f1b41d6dc7a67cb9 for context,
where the approach in this patch was suggested.

Suggested-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>