btrfs: remove spurious unlock/lock of unused_bgs_lock

Since both unused block groups and reclaim bgs lists are protected by
unused_bgs_lock then free them in the same critical section without
doing an extra unlock/lock pair.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NNikolay Borisov <nborisov@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -3911,9 +3911,7 @@ int btrfs_free_block_groups(struct btrfs_fs_info *info)
 		list_del_init(&block_group->bg_list);
 		btrfs_put_block_group(block_group);
 	}
-	spin_unlock(&info->unused_bgs_lock);
 
-	spin_lock(&info->unused_bgs_lock);
 	while (!list_empty(&info->reclaim_bgs)) {
 		block_group = list_first_entry(&info->reclaim_bgs,
 					       struct btrfs_block_group,