CVE-2025-37856

btrfs: harden block_group::bg_list against list_del() races

As far as I can tell, these calls of list_del_init() on bg_list cannot
run concurrently with btrfs_mark_bg_unused() or btrfs_mark_bg_to_reclaim(),
as they are in transaction error paths and situations where the block
group is readonly.

However, if there is any chance at all of racing with mark_bg_unused(),
or a different future user of bg_list, better to be safe than sorry.

Otherwise we risk the following interleaving (bg_list refcount in parens)

T1 (some random op) T2 (btrfs_mark_bg_unused)
!list_empty(&bg->bg_list); (1)
list_del_init(&bg->bg_list); (1)
list_move_tail (1)
btrfs_put_block_group (0)
btrfs_delete_unused_bgs
bg = list_first_entry
list_del_init(&bg->bg_list);
btrfs_put_block_group(bg); (-1)

Ultimately, this results in a broken ref count that hits zero one deref
early and the real final deref underflows the refcount, resulting in a WARNING.