imsm: don't skip resync when an invalid ppl header is found

imsm: don't skip resync when an invalid ppl header is found

If validate_ppl_imsm() detects an invalid ppl header it will be
overwritten with a valid, empty ppl header. But if we are assembling an
array after unclean shutdown this will cause the kernel to skip resync
after ppl recovery. We don't want that because if there was an invalid
ppl it's best to assume that the ppl recovery is not enough to make the
array consistent and a full resync should be performed. So when
overwriting the invalid ppl add one ppl_header_entry with a wrong
checksum. This will prevent the kernel from skipping resync after ppl
recovery.

Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>