libnvdimm/labels: Add isetcookie validation helper

In preparation to handle CXL labels with the same code that handles EFI
labels, add a specific interleave-set-cookie validation helper
rather than a getter since the CXL label type does not support this
concept. The answer for CXL labels will always be true.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162982113550.1124374.206762177785773038.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>