scsi_debug: allow huge transfer length for read/write commands

This change enables to test read/write commands with huge transfer
length such as 1GB.  For example:

	# modprobe scsi_debug dev_size_mb=1024 clustering=1 opts=1
	# cat /sys/block/$DEV/queue/max_hw_sectors_kb > \
		/sys/block/$DEV/queue/max_sectors_kb
	# fio --name=test --rw=write --bs=1g --size=1g --filename=/dev/$DEV \
		--mem=mmaphuge  --direct=1

The data type of max_sectors in scsi_host_template has been extended
to unsigned int by the previous change.  So we can increase it from
0xffff to 0xffffffff to allow such huge transfer length.

Also, this increases sg_tablesize and max_segment_size, otherwise the
maximum transfer length is limited to 64MB.
(sg_tablesize * max_segment_size = 256 * 256KB)

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