can: slcan: use scnprintf() as a hardening measure

The snprintf() function returns the number of bytes which *would* have
been copied if there were no space. So, since this code does not check
the return value, there if the buffer was not large enough then there
would be a buffer overflow two lines later when it does:

	actual = sl->tty->ops->write(sl->tty, sl->xbuff, n);

Use scnprintf() instead because that returns the number of bytes which
were actually copied.

Fixes: 52f9ac8 ("can: slcan: allow to send commands to the adapter")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/all/YsVA9KoY/ZSvNGYk@kili
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/slcan/slcan-core.c

@@ -647,7 +647,7 @@ static int slcan_transmit_cmd(struct slcan *sl, const unsigned char *cmd)
 		return -ENODEV;
 	}
 
-	n = snprintf(sl->xbuff, sizeof(sl->xbuff), "%s", cmd);
+	n = scnprintf(sl->xbuff, sizeof(sl->xbuff), "%s", cmd);
 	set_bit(TTY_DO_WRITE_WAKEUP, &sl->tty->flags);
 	actual = sl->tty->ops->write(sl->tty, sl->xbuff, n);
 	sl->xleft = n - actual;