accessibility: speakup: refactor deprecated strncpy

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

Let's refactor this function to just use synth_write().

Link: 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: [e-mail]
Signed-off-by: Justin Stitt <[e-mail]>
Suggested-by: Kees Cook <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Tested-by: Samuel Thibault <[e-mail]>
Link: https://lore.kernel.org/r/20230918-st[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/accessibility/speakup/kobjects.c

···
 				  struct kobj_attribute *attr,
 				  const char *buf, size_t count)
 {
-	u_char tmp[256];
-	int len;
-	int bytes;
-	const char *ptr = buf;
+	char *unescaped;
 	unsigned long flags;
 
 	if (!synth)
 		return -EPERM;
 
-	len = strlen(buf);
+	unescaped = kstrdup(buf, GFP_KERNEL);
+	if (!unescaped)
+		return -ENOMEM;
+
+	string_unescape_any_inplace(unescaped);
+
 	spin_lock_irqsave(&speakup_info.spinlock, flags);
-	while (len > 0) {
-		bytes = min_t(size_t, len, 250);
-		strncpy(tmp, ptr, bytes);
-		tmp[bytes] = '\0';
-		string_unescape_any_inplace(tmp);
-		synth_printf("%s", tmp);
-		ptr += bytes;
-		len -= bytes;
-	}
+	synth_write(unescaped, strlen(unescaped));
 	spin_unlock_irqrestore(&speakup_info.spinlock, flags);
+
+	kfree(unescaped);
+
 	return count;
 }