scsi: switch scsi_bios_ptable() and scsi_partsize() to gendisk

scsi: switch scsi_bios_ptable() and scsi_partsize() to gendisk

Both helpers are reading the partition table of the disk specified
by block_device of some partition on it; result depends only upon
the disk in question, so we might as well pass the struct gendisk
instead.

Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>