Bug 2394634 (CVE-2025-39759)

In the Linux kernel, the following vulnerability has been resolved:

btrfs: qgroup: fix race between quota disable and quota rescan ioctl

There's a race between a task disabling quotas and another running the
rescan ioctl that can result in a use-after-free of qgroup records from
the fs_info->qgroup_tree rbtree.

This happens as follows:

1) Task A enters btrfs_ioctl_quota_rescan() -> btrfs_qgroup_rescan();

2) Task B enters btrfs_quota_disable() and calls
   btrfs_qgroup_wait_for_completion(), which does nothing because at that
   point fs_info->qgroup_rescan_running is false (it wasn't set yet by
   task A);

3) Task B calls btrfs_free_qgroup_config() which starts freeing qgroups
   from fs_info->qgroup_tree without taking the lock fs_info->qgroup_lock;

4) Task A enters qgroup_rescan_zero_tracking() which starts iterating
   the fs_info->qgroup_tree tree while holding fs_info->qgroup_lock,
   but task B is freeing qgroup records from that tree without holding
   the lock, resulting in a use-after-free.

Fix this by taking fs_info->qgroup_lock at btrfs_free_qgroup_config().
Also at btrfs_qgroup_rescan() don't start the rescan worker if quotas
were already disabled.