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/utf8n.h

@@ -39,7 +39,6 @@ extern const struct utf8data *utf8nfdicf(unsigned int maxage);
  * Returns 0 if only ignorable code points are present.
  * Returns -1 if the input is not valid UTF-8.
  */
-extern ssize_t utf8len(const struct utf8data *data, const char *s);
 extern ssize_t utf8nlen(const struct utf8data *data, const char *s, size_t len);
 
 /* Needed in struct utf8cursor below. */