btrfs-progs: print-tree: Do correct offset output for ROOT_ITEM

btrfs-progs: print-tree: Do correct offset output for ROOT_ITEM

Commit Fixes: 8c36786c8198 ("btrfs-progs: print-tree: Print offset as
tree objectid for ROOT_ITEM") changes how we translate offset of
ROOT_ITEM.

However the fact is, even for ROOT_ITEM, we have different meaning of
offset.

For tree reloc tree, it's indeed subvolume id.  But for other trees,
it's the transid of when it's created.

Reported-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>