libnvdimm/labels: Add blk special cases for nlabel and position helpers

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.

Finish off the BLK-mode specific helper conversion with the nlabel and
position behaviour that is specific to EFI v1.2 labels and not the
original v1.1 definition.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/162982115698.1124374.10182273478536799613.stgit@dwillia2-desk3.amr.corp.intel.com


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