btrfs: fix wrong max system array size check in kernel space

For system chunk array,
We copy a "disk_key" and an chunk item each time,
so there should be enough space to hold both of them,
not only the chunk item.
Signed-off-by: NGui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/volumes.c

@@ -3921,7 +3921,8 @@ static int btrfs_add_system_chunk(struct btrfs_root *root,
 	u8 *ptr;
 
 	array_size = btrfs_super_sys_array_size(super_copy);
-	if (array_size + item_size > BTRFS_SYSTEM_CHUNK_ARRAY_SIZE)
+	if (array_size + item_size + sizeof(disk_key)
+			> BTRFS_SYSTEM_CHUNK_ARRAY_SIZE)
 		return -EFBIG;
 
 	ptr = super_copy->sys_chunk_array + array_size;