libnvdimm/labels: Add a checksum calculation helper

libnvdimm/labels: Add a checksum calculation helper

In preparation for LIBNVDIMM to manage labels on CXL devices deploy
helpers that abstract the label type from the implementation. The CXL
label format is mostly similar to the EFI label format with concepts /
fields added, like dynamic region creation and label type guids, and
other concepts removed like BLK-mode and interleave-set-cookie ids.

CXL labels support checksums by default, but early versions of the EFI
labels did not. Add a validate function that can return true in the case
the label format does not implement a checksum.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162982114637.1124374.6966639787307077105.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>