libnvdimm, namespace: record 'lbasize' for pmem namespaces

Commit f979b13c "libnvdimm, label: honor the lba size specified in
v1.2 labels") neglected to update the 'lbasize' in the label when the
namespace sector_size attribute was written. We need this value in the
label for inter-OS / pre-OS compatibility.

Fixes: f979b13c

 ("libnvdimm, label: honor the lba size specified in v1.2 labels")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>