tty: n_gsm: replace deprecated strncpy with strscpy

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

We expect nc->if_name to be NUL-terminated based on existing manual
NUL-byte assignments and checks:
|	nc.if_name[IFNAMSIZ-1] = '\0';
...
| 	if (nc->if_name[0] != '\0')

Let's use the new 2-argument strscpy() since it guarantees
NUL-termination on the destination buffer while correctly using the
destination buffers size to bound the operation.

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: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20240318-strncpy-drivers-tty-n_gsm-c-v1-1-da37a07c642e@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -4010,7 +4010,7 @@ static int gsm_create_network(struct gsm_dlci *dlci, struct gsm_netconfig *nc)
 	mux_net = netdev_priv(net);
 	mux_net->dlci = dlci;
 	kref_init(&mux_net->ref);
-	strncpy(nc->if_name, net->name, IFNAMSIZ); /* return net name */
+	strscpy(nc->if_name, net->name); /* return net name */
 
 	/* reconfigure dlci for network */
 	dlci->prev_adaption = dlci->adaption;