imsm: fix max disks per array

imsm: fix max disks per array

Validate geometry is incorrectly looking at max disks support which is
irrelevant for md/mdadm.  ->dpa (disks per array) is how many disks the
orom will allow per volume.

Also cleanup an unnecessary ->orom check, is_raid_level_supported()
already does the right thing in the !orom case.

Cc: Marcin Labun <marcin.labun@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>