nvdimm: Use str_plural() to simplify the code

Use the string choice helper function str_plural() to simplify the code.

Signed-off-by: Xichao Zhao <zhao.xichao@vivo.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>