tty/vt: UTF-8 parsing update according to RFC 3629, modern Unicode

vc_translate_unicode() and vc_sanitize_unicode() parse input to the
UTF-8-enabled console, marking invalid byte sequences and producing Unicode
codepoints. The current algorithm follows ancient Unicode and may accept
invalid byte sequences, pass on non-existent codepoints and reject valid
sequences.

The patch restores the functions' compliance with modern Unicode (v15.1 [1]
+ many previous versions) as well as RFC 3629 [2].
1. Codepoint space is limited to 0x10FFFF.
2. "Noncharacters", such as U+FFFE, U+FFFF, are no longer invalid in
   Unicode and will be accepted. Another option was to complete the set of
   noncharacters (used to be just those two, now there's more) and preserve
   the rejection step. This is indeed what Unicode suggests ([1] chap.
   23.7) (not requires), but most codepoints are !iswprint(), so selecting
   just the noncharacters seemed arbitrary and futile (and unnecessary).

This is not a security patch. I'm not aware of any present security
implications of the old code.

[1] https://www.unicode.org/versions/Unicode15.1.0
[2] https://datatracker.ietf.org/doc/html/rfc3629Signed-off-by: Roman Žilka <roman.zilka@gmail.com>
Link: https://lore.kernel.org/r/598ab459-6ba9-4a17-b4a1-08f26a356fc0@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vt.c

@@ -2637,7 +2637,7 @@ static inline int vc_translate_ascii(const struct vc_data *vc, int c)
  */
 static inline int vc_sanitize_unicode(const int c)
 {
-	if ((c >= 0xd800 && c <= 0xdfff) || c == 0xfffe || c == 0xffff)
+	if (c >= 0xd800 && c <= 0xdfff)
 		return 0xfffd;
 
 	return c;
@@ -2656,10 +2656,7 @@ static inline int vc_sanitize_unicode(const int c)
  */
 static int vc_translate_unicode(struct vc_data *vc, int c, bool *rescan)
 {
-	static const u32 utf8_length_changes[] = {
-		0x0000007f, 0x000007ff, 0x0000ffff,
-		0x001fffff, 0x03ffffff, 0x7fffffff
-	};
+	static const u32 utf8_length_changes[] = {0x7f, 0x7ff, 0xffff, 0x10ffff};
 
 	/* Continuation byte received */
 	if ((c & 0xc0) == 0x80) {
@@ -2705,14 +2702,7 @@ static int vc_translate_unicode(struct vc_data *vc, int c, bool *rescan)
 	} else if ((c & 0xf8) == 0xf0) {
 		vc->vc_utf_count = 3;
 		vc->vc_utf_char = (c & 0x07);
-	} else if ((c & 0xfc) == 0xf8) {
-		vc->vc_utf_count = 4;
-		vc->vc_utf_char = (c & 0x03);
-	} else if ((c & 0xfe) == 0xfc) {
-		vc->vc_utf_count = 5;
-		vc->vc_utf_char = (c & 0x01);
 	} else {
-		/* 254 and 255 are invalid */
 		return 0xfffd;
 	}