imsm: update num_data_stripes according to dev_size

imsm: update num_data_stripes according to dev_size

If array was created in UEFI there is possibility that
member size is not rounded to 1MB. After any size reconfiguration
it will be rounded down to 1MB per each member but the old
component size will remain in metadata.
During reshape old array size is calculated from component size because
dev_size is not a part of map and is bumped to new value quickly.
It may result in size mismatch if array is assembled during reshape.

If difference in calculated size and dev_size is observed try to fix it.
num_data_stripes value can be safety updated to smaller value if array
doesn't occuppy whole reserved component space.

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