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.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>