Btrfs: fix a regression in balance usage filter

Commit 3fed40cc ("Btrfs: cleanup duplicated division functions"), which
was merged into 3.8-rc1, has introduced a regression by removing logic
that was guarding us against bad user input.  Bring it back.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/volumes.c

@@ -2614,7 +2614,14 @@ static int chunk_usage_filter(struct btrfs_fs_info *fs_info, u64 chunk_offset,
 	cache = btrfs_lookup_block_group(fs_info, chunk_offset);
 	chunk_used = btrfs_block_group_used(&cache->item);
 
-	user_thresh = div_factor_fine(cache->key.offset, bargs->usage);
+	if (bargs->usage == 0)
+		user_thresh = 0;
+	else if (bargs->usage > 100)
+		user_thresh = cache->key.offset;
+	else
+		user_thresh = div_factor_fine(cache->key.offset,
+					      bargs->usage);
+
 	if (chunk_used < user_thresh)
 		ret = 0;