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

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SUSE CVE-2025-37856

In the Linux kernel, the following vulnerability has been resolved: btrfs: harden blockgroup::bglist against listdel races As far as I can tell, these calls of listdelinit on bglist cannot run concurrently with btrfsmarkbgunused or btrfsmarkbgtoreclaim, as they are in transaction error paths and...