imsm: add support for literal RAID 10

imsm: add support for literal RAID 10

As for now, IMSM supports only 4 drive RAID 1+0. This patch is first in
series to add support for literal RAID 10 (with more than 4 drives) to
imsm.

Allow setting RAID 10 as raid level for imsm arrays.

Add update_imsm_raid_level() to handle raid level updates. Set RAID10 as
default level for imsm R0 to R10 migrations. Replace magic numbers with
defined values for RAID level checks/assigns.

Signed-off-by: Mateusz Kusiak <mateusz.kusiak@intel.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>