libnvdimm, label: fix index block size calculation

The old calculation assumed that the label space was 128k and the label
size is 128. With v1.2 labels where the label size is 256 this
calculation will return zero. We are saved by the fact that the
nsindex_size is always pre-initialized from a previous 128 byte
assumption and we are lucky that the index sizes turn out the same.

Fix this going forward in case we start encountering different
geometries of label areas besides 128k.

Since the label size can change from one call to the next, drop the
caching of nsindex_size.

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