Btrfs: fix to search one more bitmap for cluster setup

Suppose there are two bitmaps [0, 256], [256, 512] and one extent
[100, 120] in the free space cache, and we want to setup a cluster
with offset=100, bytes=50.

In this case, there will be only one bitmap [256, 512] in the temporary
bitmaps list, and then setup_cluster_bitmap() won't search bitmap [0, 256].

The cause is, the list is constructed in setup_cluster_no_bitmap(),
and only bitmaps with bitmap_entry->offset >= offset will be added
into the list, and the very bitmap that convers offset has
bitmap_entry->offset <= offset.
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/free-space-cache.c

@@ -2453,10 +2453,22 @@ setup_cluster_bitmap(struct btrfs_block_group_cache *block_group,
 	struct btrfs_free_space *entry;
 	struct rb_node *node;
 	int ret = -ENOSPC;
+	u64 bitmap_offset = offset_to_bitmap(ctl, offset);
 
 	if (ctl->total_bitmaps == 0)
 		return -ENOSPC;
 
+	/*
+	 * The bitmap that covers offset won't be in the list unless offset
+	 * is just its start offset.
+	 */
+	entry = list_first_entry(bitmaps, struct btrfs_free_space, list);
+	if (entry->offset != bitmap_offset) {
+		entry = tree_search_offset(ctl, bitmap_offset, 1, 0);
+		if (entry && list_empty(&entry->list))
+			list_add(&entry->list, bitmaps);
+	}
+
 	/*
 	 * First check our cached list of bitmaps and see if there is an entry
 	 * here that will work.