generic/108: use sysfs values for logical,physical block size in scsi_debug

generic/108: use sysfs values for logical,physical block size in scsi_debug

scsi_debug device used for test, is created with assumption of 512
bytes logical and physical block size.

This causes error in lvcreate step, when SCRATCH_DEV device lba is
not 512 bytes. This can be solved by reading block size from sysfs
of device.  If sysfs is missing fallback to 512 bytes as default.

Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>