btrfs: Free fully occupied bitmap in cluster

If there's no more free space in a bitmap, we should free it.
Reviewed-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

fs/btrfs/free-space-cache.c

@@ -1788,6 +1788,8 @@ static u64 btrfs_alloc_from_bitmap(struct btrfs_block_group_cache *block_group,
 
 	ret = search_start;
 	bitmap_clear_bits(block_group, entry, ret, bytes);
+	if (entry->bytes == 0)
+		free_bitmap(block_group, entry);
 out:
 	spin_unlock(&cluster->lock);
 	spin_unlock(&block_group->tree_lock);