btrfs: switch order of unlocks of space_info and bg in do_trimming()

In function do_trimming(), block_group->lock should be unlocked first,
as the locks should be released in the reverse order. This does not
cause problems but should follow the best practices.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NSu Yue <suy.fnst@cn.fujitsu.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/free-space-cache.c

@@ -3166,8 +3166,8 @@ static int do_trimming(struct btrfs_block_group_cache *block_group,
 			space_info->bytes_readonly += reserved_bytes;
 		block_group->reserved -= reserved_bytes;
 		space_info->bytes_reserved -= reserved_bytes;
-		spin_unlock(&space_info->lock);
 		spin_unlock(&block_group->lock);
+		spin_unlock(&space_info->lock);
 	}
 
 	return ret;