unicode: simplify utf8len

Just use the utf8nlen implementation with a (size_t)-1 len argument,
similar to utf8_lookup.  Also move the function to utf8-selftest.c, as
it isn't used anywhere else.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>

fs/unicode/utf8-selftest.c

@@ -160,6 +160,11 @@ static const struct {
 	}
 };
 
+static ssize_t utf8len(const struct utf8data *data, const char *s)
+{
+	return utf8nlen(data, s, (size_t)-1);
+}
+
 static void check_utf8_nfdi(void)
 {
 	int i;