fs/nls: Fix inconsistency between utf8_to_utf32() and utf32_to_utf8()

After commit 25524b619029 ("fs/nls: Fix utf16 to utf8 conversion"),
the return values of utf8_to_utf32() and utf32_to_utf8() are
inconsistent when encountering an error: utf8_to_utf32() returns -1,
while utf32_to_utf8() returns errno codes. Fix this inconsistency
by modifying utf8_to_utf32() to return errno codes as well.

Fixes: 25524b619029 ("fs/nls: Fix utf16 to utf8 conversion")
Suggested-by: Andy Shevchenko <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Armin Wolf <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Reviewed-by: Ilpo Järvinen <[e-mail]>
Signed-off-by: Ilpo Järvinen <[e-mail]>

fs/nls/nls_base.c

···
 			l &= t->lmask;
 			if (l < t->lval || l > UNICODE_MAX ||
 					(l & SURROGATE_MASK) == SURROGATE_PAIR)
-				return -1;
+				return -EILSEQ;
+
 			*pu = (unicode_t) l;
 			return nc;
 		}
 		if (inlen <= nc)
-			return -1;
+			return -EOVERFLOW;
+
 		s++;
 		c = (*s ^ 0x80) & 0xFF;
 		if (c & 0xC0)
-			return -1;
+			return -EILSEQ;
+
 		l = (l << 6) | c;
 	}
-	return -1;
+	return -EILSEQ;
 }
 EXPORT_SYMBOL(utf8_to_utf32);