imsm: FIX: Function rework - imsm_count_failed()

imsm: FIX: Function rework - imsm_count_failed()

imsm_count_failed() assumes that on the same positions in both maps
the same disk indexes are kept. This is not always true /e.g. rebuild/
It can occur that disk taken for rebuild fails at once.
Degradation on the same positions in both maps refers to different disks.
Sum of both ords can point on not failed disk. This can cause wrong
failed disk counting.

Check both maps independently. This allows for getting real degradation
information.

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>