imsm: FIX: Chunk size migration is not possible

imsm: FIX: Chunk size migration is not possible

Chunk size only migration for raid0 and raid5 is not possible.
(mdadm UT 15* fails). Mdadm exits with information:
   mdadm: imsm unknown layout 0xffffffff for this raid level 0

Problem was introduced in patch (2011-11-16):
   imsm: platform capabilities are not validated during level migration

During chunk size migration layout variable is not set correctly.
Set it correctly for this migration type.

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