imsm: FIX: size have to be calculated based on first map

imsm: FIX: size have to be calculated based on first map

Before reshape finalization migration is still present in metadata.
After patch 'imsm: FIX: crash during getting map'
function get_imsm_map() returns correct value,
this means in our case from second (start) map.

We should calculate map size basing on first (final) map.
For this we should request it by setting second function parameter to '0'

Signed-off-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>