clippy: Fix clippy version 0.1.60

Fix clippy lints from clippy 0.1.60 (7737e0b 2022-04-04).

Signed-off-by: Felix Obenhuber <felix@obenhuber.de>

tests/memory.rs

@@ -23,7 +23,7 @@ fn test_disable_oom_killer() {
         if !mem_controller.v2() {
             // disable oom killer
             let r = mem_controller.disable_oom_killer();
-            assert!(!r.is_err());
+            assert!(r.is_ok());
 
             // after disable
             let m = mem_controller.memory_stat();