FIX: Raid0 expansion cannot be restarted

FIX: Raid0 expansion cannot be restarted

When raid0 expansion is restarted, mdadm refuses to correctly assemble
array because critical section cannot be restored from backup file.
mdadm exits with information:
	mdadm: Failed to restore critical section for reshape - sorry.

For raid0 new level is 0, current array level is 4.
Function Grow_restart() doesn't allow for level change.

Grow_restart really shouldn't be checking for level changes.
As they are always instantaneous they should never appear
in the metadata so it doesn't mean anything to check for them.

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