imsm: fix: correct adding and activation of spare disks

imsm: fix: correct adding and activation of spare disks

During activation of spare disks, only one of all available
spare disks can be activated at this moment.
It causes that for example during take-over from
RAID0 with 2 disks to RAID10, only one of two spare disks
is taken for recovery and a degraded RAID10 array
with only 3 of 4 working disks is created.
It has been fixed by adding more than one of all available
spare disks and saving them in additional_test_list
which is passed to imsm_add_spare().

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