imsm: fix: correct printing value of blocks per migration unit

imsm: fix: correct printing value of blocks per migration unit

The value of blocks per migration unit is not printed correctly
when the metadata's content is examined using -E option on disks
without present migration record. (Migration record is present only
on 2 first disks in array due to IMSM compatibility restrictions.)

Printing the value of blocks per migration unit was corrected.
It is printed as N/A (Not Available) for disks
without the migration record.

Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>