imsm: fix: Fixes metadata after migration from Raid 0 to Raid 10

After migration from Raid 0 to Raid 10, the metadata is incorrect,
leaving one mirror disk marked as spare and one missing disk as a member
of the array.
The reason is that the metadata update code for spare activation
procedure takes into account one spare disk only, not checking
the following ones.

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