scsi: increase upper limit for max_sectors

max_sectors in struct Scsi_Host specifies maximum number of sectors
allowed in a single SCSI command.  The data type of max_sectors is
unsigned short, so the maximum transfer length per SCSI command is
limited to less than 256MB in 4096-bytes sector size. (0xffff * 4096)

This commit increases the SCSI mid level's limitation for max_sectors
upto the block layer's limitation for max_hw_sectors by extending the
data type of max_sectors in struct Scsi_Host and scsi_host_template,
so that SCSI lower level drivers can specify more than 0xffff.

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>