libnvdimm/label: Add a helper for nlabel validation

In the CXL namespace label there is no need for nlabel since that is
inferred from the region. Add a helper that moves nsl_get_label() behind
a helper that validates the number of labels relative to the region.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163116430293.2460985.12693942353621355232.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>