dm crypt: fix endianness annotations around org_sector_of_dmreq

The sector used here is a little endian value, so use the right
type for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>