scsi: sg: factor out sg_fill_request_table()

Factor out sg_fill_request_table() for better readability.

[mkp: typos, applied by hand]

Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>