scsi: rename SG related struct and functions

Rename SCSI specific struct and functions to more genenic names.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lin <ming.l@ssi.samsung.com>
Reviewed-by: Sagi Grimberg <sgi@grimberg.me>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>