dax: refactor deprecated strncpy

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

We should prefer more robust and less ambiguous string interfaces.

`dax_id->dev_name` is expected to be NUL-terminated and has been zero-allocated.

A suitable replacement is `strscpy` [2] due to the fact that it
guarantees NUL-termination on the destination buffer. Moreover, due to
`dax_id` being zero-allocated the padding behavior of `strncpy` is not
needed and a simple 1:1 replacement of strncpy -> strscpy should
suffice.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: KSPP#90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>

Author: JustinStitt

drivers/dax/bus.c

@@ -103,7 +103,7 @@ static ssize_t do_id_store(struct device_driver *drv, const char *buf,
 		if (action == ID_ADD) {
 			dax_id = kzalloc(sizeof(*dax_id), GFP_KERNEL);
 			if (dax_id) {
-				strncpy(dax_id->dev_name, buf, DAX_NAME_LEN);
+				strscpy(dax_id->dev_name, buf, DAX_NAME_LEN);
 				list_add(&dax_id->list, &dax_drv->ids);
 			} else
 				rc = -ENOMEM;