net: dsa: remove deprecated strncpy

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

Even call sites utilizing length-bounded destination buffers should
switch over to using `strtomem` or `strtomem_pad`. In this case,
however, the compiler is unable to determine the size of the `data`
buffer which renders `strtomem` unusable. Due to this, `strscpy`
should be used.

It should be noted that most call sites already zero-initialize the
destination buffer. However, I've opted to use `strscpy_pad` to maintain
the same exact behavior that `strncpy` produced (zero-padded tail up to
`len`).

Also see [3].

[1]: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings
[2]: elixir.bootlin.com/linux/v6.3/source/net/ethtool/ioctl.c#L1944
[3]: manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html

Link: KSPP#90
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>

net/dsa/slave.c

@@ -21,6 +21,7 @@
 #include <linux/if_hsr.h>
 #include <net/dcbnl.h>
 #include <linux/netpoll.h>
+#include <linux/string.h>
 
 #include "dsa.h"
 #include "port.h"
@@ -1056,10 +1057,10 @@ static void dsa_slave_get_strings(struct net_device *dev,
 	if (stringset == ETH_SS_STATS) {
 		int len = ETH_GSTRING_LEN;
 
-		strncpy(data, "tx_packets", len);
-		strncpy(data + len, "tx_bytes", len);
-		strncpy(data + 2 * len, "rx_packets", len);
-		strncpy(data + 3 * len, "rx_bytes", len);
+		strscpy_pad(data, "tx_packets", len);
+		strscpy_pad(data + len, "tx_bytes", len);
+		strscpy_pad(data + 2 * len, "rx_packets", len);
+		strscpy_pad(data + 3 * len, "rx_bytes", len);
 		if (ds->ops->get_strings)
 			ds->ops->get_strings(ds, dp->index, stringset,
 					     data + 4 * len);