btrfs: push lookup_info into struct walk_control

Instead of using a flag we're passing around everywhere, add a field to
walk_control that we're already passing around everywhere and use that
instead.

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