libnvdimm, namespace: Replace kmemdup() with kstrndup()

kstrndup() takes care of '\0' terminator for the strings.

Use it here instead of kmemdup() + explicit terminating the input string.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>