[SCSI] sd: Move sd.h header file

Christoph objected to having sd.h in include/scsi since it is internal
to the sd driver.  Move it to drivers/scsi/sd.h.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>