nvdimm: Clarify comment in sizeof_namespace_index

When working on the label code I found it rather confusing to see several
spots that reference a minimum label size of 256 while working with labels
that are 128 bytes in size.

This patch is meant to provide a clarification on one of the comments that
was at the heart of the issue. Specifically for version 1.2 and later of
the namespace specification the minimum label size is 256, prior to that
the minimum label size was 128. So we should state that as such to avoid
confusion.

Reviewed-by: Toshi Kani <toshi.kani@hpe.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>