rt: fix default thread number logic (#2457)

Previously, the function picking the default number of threads for the
threaded runtime did not factor in `max_threads`. Instead, it only used
the value returned by `num_cpus`. However, if `num_cpus` returns a value
greater than `max_threads`, then the function would panic.

This patch fixes the function by limiting the default number of threads
by `max_threads`.

Fixes #2452

tokio/tests/rt_threaded.rs

@@ -322,6 +322,16 @@ fn multi_threadpool() {
     done_rx.recv().unwrap();
 }
 
+// Testing this does not panic
+#[test]
+fn max_threads() {
+    let _rt = tokio::runtime::Builder::new()
+        .threaded_scheduler()
+        .max_threads(1)
+        .build()
+        .unwrap();
+}
+
 fn rt() -> Runtime {
     Runtime::new().unwrap()
 }