btrfs: add boilerplate code for directly including the crypto framework

Add boilerplate code for directly including the crypto framework.  This
helps us flipping the switch for new algorithms.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>