nvdimm: use the right length of "pmem"

In order to test that the name of a resource begins with "pmem", call
strncmp() with 4 as length instead of 3 to match the whole prefix.
Signed-off-by: NNicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/nvdimm/label.c

@@ -938,7 +938,7 @@ int nd_pmem_namespace_label_update(struct nd_region *nd_region,
 		}
 
 		for_each_dpa_resource(ndd, res)
-			if (strncmp(res->name, "pmem", 3) == 0)
+			if (strncmp(res->name, "pmem", 4) == 0)
 				count++;
 		WARN_ON_ONCE(!count);