imsm: Write empty PPL header if assembling regular clean array.

imsm: Write empty PPL header if assembling regular clean array.

If array was initially assembled with kernel without PPL support -
initial header was never written to the drive.
If initial resync was completed and system is rebooted to kernel with
PPL support - mdadm prevents from assembling normal clean array
due to lack of valid PPL.
Write empty header when assemble normal clean array, so the
its assamble is no longer blocked.

Signed-off-by: Pawel Baldysiak <pawel.baldysiak@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>