imsm: fix: add support for OLCE and migration to imsm_count_failed
The problem occurs when array under OLCE (from 3 to 6 disks)
is assembled incrementally. Mdadm tries to start array
just after adding the third disk (this is equal to the number of disks
before the start of reshape). It does not succeed,
the volume does not assembly correctly.
The function counting failed disks (imsm_count_failed())
was fixed for migration case. Now all disk members in both maps
are checked when failed disks are counted correctly.