imsm: do not use blocks_per_member in array size calculations

imsm: do not use blocks_per_member in array size calculations

mdadm assumes that blocks_per_member value is equal to num_data_stripes *
blocks_per_stripe but it is not true. For IMSM arrays created in OROM
NUM_BLOCKS_DIRTY_STRIPE_REGION sectors are added up to this value. Because
of this mdadm shows invalid size of arrays created in OROM and to fix this
we need to use array size calculation based on num data stripes and blocks
per stripe.

Signed-off-by: Mariusz Dabrowski <mariusz.dabrowski@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>