scsi: sg: update comment for blk_get_request()

The calling convention of blk_get_request() has changed in lk 4.18; update
the comment in sg.c to match.

Fixes: ff005a06

 ("block: sanitize blk_get_request calling conventions")
Signed-off-by: Tony Battersby <tonyb@cybernetics.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>