kernel: futex: Allow for fast return on k_futex_wake()

Updates k_futex_wake() to allow for a fast return path when no threads
were waiting.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>

kernel/futex.c

@@ -47,7 +47,11 @@ int z_impl_k_futex_wake(struct k_futex *futex, bool wake_all)
 		}
 	} while (thread && wake_all);
 
-	z_reschedule(&futex_data->lock, key);
+	if (woken == 0) {
+		k_spin_unlock(&futex_data->lock, key);
+	} else {
+		z_reschedule(&futex_data->lock, key);
+	}
 
 	return woken;
 }