lockd: Eliminate harmless mixed sign comparison in nlmdbg_cookie2a()

The cookie->len field is unsigned, so the loop index variable in
nlmdbg_cookie2a() should also be unsigned.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/lockd/xdr.c

@@ -612,8 +612,7 @@ const char *nlmdbg_cookie2a(const struct nlm_cookie *cookie)
 	 * called with BKL held.
 	 */
 	static char buf[2*NLM_MAXCOOKIELEN+1];
-	int i;
-	int len = sizeof(buf);
+	unsigned int i, len = sizeof(buf);
 	char *p = buf;
 
 	len--;	/* allow for trailing \0 */