btrfs: move comment in find_parent_nodes()

This comment was much closer to the related code when it was originally
added, but has slowly migrated north far from its ancestral lands.  Move
it back down with its people.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>