libnvdimm: allow a platform to force enable label support

Platforms like QEMU-KVM implement an NFIT table and label DSMs.
However, since that environment does not define an aliased
configuration, the labels are currently ignored and the kernel registers
a single full-sized pmem-namespace per region. Now that the kernel
supports sub-divisions of pmem regions the labels have a purpose.
Arrange for the labels to be honored when we find an existing / valid
namespace index block.

Cc: <qemu-devel@nongnu.org>
Cc: Haozhong Zhang <haozhong.zhang@intel.com>
Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>