imsm: set array size at Create/Assemble

imsm: set array size at Create/Assemble

imsm arrays round down the effective array size to the closest 1
megabyte boundary so teach get_info_super_imsm and sysfs_set_array to
set 'md/array_size' if available (and make sure ddf uses the default
size).

Signed-off-by: Dan Williams <dan.j.williams@intel.com>