btrfs: defrag: enable defrag for subpage case

With the new infrastructure which has taken subpage into consideration,
now we should be safe to allow defrag to work for subpage case.

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