policy.c: Avoid to take spare without defined domain by imsm

policy.c: Avoid to take spare without defined domain by imsm

Only Imsm get_disk_controller_domain returns disk controller domain for
each disk. It causes that mdadm automatically creates disk controller
domain policy for imsm metadata, and imsm containers in the same disk
controller domain can take spare for recovery.

Ignore spares if only one imsm domain is matched.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>