scsi: block: Add a queue_max_bytes() helper

Return the max_sectors value in bytes.  Lifted from scsi_ioctl.c.

Link: https://lore.kernel.org/r/20210724072033.1284840-13-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>