sd: use READ_16 or WRITE_16 when transfer length is greater than 0xffff

This change makes the scsi disk driver handle the requests whose
transfer length is greater than 0xffff with READ_16 or WRITE_16.

However, this is a preparation for extending the data type of
max_sectors in struct Scsi_Host and scsi_host_template.  So, it is
impossible to happen this condition for now, because SCSI low-level
drivers can not specify max_sectors greater than 0xffff due to the
data type limitation.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>