Btrfs: Add root tree pointer transaction ids

This patch adds transaction IDs to root tree pointers.
Transaction IDs in tree pointers are compared with the
generation numbers in block headers when reading root
blocks of trees. This can detect some types of IO errors.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>

fs/btrfs/ioctl.c

@@ -108,6 +108,7 @@ static noinline int create_subvol(struct btrfs_root *root,
 	inode_item->mode = cpu_to_le32(S_IFDIR | 0755);
 
 	btrfs_set_root_bytenr(&root_item, leaf->start);
+	btrfs_set_root_generation(&root_item, trans->transid);
 	btrfs_set_root_level(&root_item, 0);
 	btrfs_set_root_refs(&root_item, 1);
 	btrfs_set_root_used(&root_item, 0);