CVE-2026-43046 btrfs: reject root items with drop_progress and zero drop_level

In the Linux kernel, the following vulnerability has been resolved: btrfs: reject root items with dropprogress and zero droplevel BUG When recovering relocation at mount time, mergerelocroot and btrfsdropsnapshot both use BUGONlevel == 0 to guard against an impossible state: a non-zero dropprogre...