sync: add_permits does not panic with usize::MAX >> 3

Fixed incorrect shift in the assignment of permits on batch_semaphore.rs

Fixes: #3095

tokio/src/sync/batch_semaphore.rs

@@ -253,7 +253,7 @@ impl Semaphore {
             }
 
             if rem > 0 && is_empty {
-                let permits = rem << Self::PERMIT_SHIFT;
+                let permits = rem >> Self::PERMIT_SHIFT;
                 assert!(
                     permits < Self::MAX_PERMITS,
                     "cannot add more than MAX_PERMITS permits ({})",

tokio/tests/sync_semaphore.rs

@@ -79,3 +79,9 @@ async fn stresstest() {
     let _p5 = sem.try_acquire().unwrap();
     assert!(sem.try_acquire().is_err());
 }
+
+#[test]
+fn add_max_amount_permits() {
+    let s = tokio::sync::Semaphore::new(0);
+    s.add_permits(usize::MAX >> 3);
+}