libnvdimm, namespace: fix pmem namespace leak, delete when size set to zero

Commit 98a29c39 ("libnvdimm, namespace: allow creation of multiple
pmem-namespaces per region") added support for establishing additional
pmem namespace beyond the seed device, similar to blk namespaces.
However, it neglected to delete the namespace when the size is set to
zero.

Fixes: 98a29c39

 ("libnvdimm, namespace: allow creation of multiple pmem-namespaces per region")
Cc: <stable@vger.kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>