[SCSI] remove the scsi_request interface from the gdth driver

Initial pass at converting the gdth driver away from the scsi_request
interface so that the request interface can be removed post 2.6.18
without breaking gdth.  Based on changes from Christoph Hellwig
<hch@lst.de>

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>