imsm: fix: Allowed to create 2 volumes with total size less then maximum.

imsm: fix: Allowed to create 2 volumes with total size less then maximum.

mdadm allows to create second volume on the same disk set, whose size is
less then the free space left in the container (with IMSM_NO_PLATFORM
undefined or set to 0). This is an OROM compatibility issue.
It is fixed by verifying whether IMSM_NO_PLATFORM is set and for
the second volume creation scenario, requested size is verified against
remaining available space.

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