scsi: core: Add helper to return number of logical blocks in a request

Link: https://lore.kernel.org/r/20210806040023.5355-2-martin.petersen@oracle.com


Cc: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>