Btrfs: allow for selecting only completely empty chunks

Enhance balance usage filter by making it possible to balance out only
completely empty chunks.  Today, usage filter properly acts on values
from 1 to 99 inclusive, usage=100 selects all chunks, and usage=0
selects no chunks.  This commit changes the usage=0 case: the new
meaning is to restripe only completely empty chunks and nothing else.

Suggested-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/volumes.c

@@ -2679,7 +2679,7 @@ static int chunk_usage_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset,
 	chunk_used = btrfs_block_group_used(&cache->item);
 
 	if (bargs->usage == 0)
-		user_thresh = 0;
+		user_thresh = 1;
 	else if (bargs->usage > 100)
 		user_thresh = cache->key.offset;
 	else