Fix flaky test TestNotifyOnOOM

Fixes: opencontainers#1228

It can be reproduced by applying this patch:
```diff
@@ -45,6 +46,7 @@ func registerMemoryEvent(cgDir string, evName string, arg string) (<-chan struct
        go func() {
                defer func() {
                        close(ch)
+                       <-time.After(1 * time.Second)
                        eventfd.Close()
                        evFile.Close()
                }()
```

We can close channel after fds were closed.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

libcontainer/notify_linux.go

@@ -44,9 +44,9 @@ func registerMemoryEvent(cgDir string, evName string, arg string) (<-chan struct
 	ch := make(chan struct{})
 	go func() {
 		defer func() {
-			close(ch)
 			eventfd.Close()
 			evFile.Close()
+			close(ch)
 		}()
 		buf := make([]byte, 8)
 		for {