dasd: cache real_sector_size in linux_probe

dasd: cache real_sector_size in linux_probe

dasd_alloc was doing an ioctl to get the real sector size for the dasd
device (we are faking 512 bytes sectors), but when called directly
from ped_disk_new_fresh(), the device is not open causing this to fail.

This patch fixes things by caching the real_sector_size in the linux
arch_specific data and making the dasd label handling use this.
libparted/arch/linux.c(_device_set_sector_size): Cache real sector size.
(_device_set_sector_size, init_dasd, linux_new): Only compile dasd code
on s390 and s390x.