libnvdimm, label: add v1.2 nvdimm label definitions

In support of improved interoperability between operating systems and pre-boot
environments the Intel proposed NVDIMM Namespace Specification [1], has been
adopted and modified to the the UEFI 2.7 NVDIMM Label Protocol [2].

Update the definitions of the namespace label data structures so that the new
format can be supported alongside the existing label format.

The new specification changes the default label size to 256 bytes, so
everywhere that relied on sizeof(struct nd_namespace_label) must now use the
sizeof_namespace_label() helper.

There should be no functional differences from these changes as the
default is still the v1.1 128-byte format. Future patches will move the
default to the v1.2 definition.

[1]: http://pmem.io/documents/NVDIMM_Namespace_Spec.pdf
[2]: http://www.uefi.org/sites/default/files/resources/UEFI_Spec_2_7.pdf



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