libnvdimm/of_pmem: Use devm_kstrdup instead of kstrdup and check its return value

Use devm_kstrdup() instead of kstrdup() and check its return value to
avoid memory leak.

Fixes: 49bddc73

 ("libnvdimm/of_pmem: Provide a unique name for bus provider")
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>