fix: linter issue

megacheck, gosimple and unused has been deprecated and subsumed by
staticcheck. And staticcheck also has been upgraded. we need to update
code for the linter issue.

close: #2945

Signed-off-by: Wei Fu <fuweid89@gmail.com>

container_linux_test.go

@@ -1120,7 +1120,7 @@ func TestDaemonRestartWithRunningShim(t *testing.T) {
 	}
 
 	pid := task.Pid()
-	if pid <= 0 {
+	if pid < 1 {
 		t.Fatalf("invalid task pid %d", pid)
 	}
 
@@ -1410,7 +1410,7 @@ func testUserNamespaces(t *testing.T, readonlyRootFS bool) {
 		t.Fatal(err)
 	}
 
-	if pid := task.Pid(); pid <= 0 {
+	if pid := task.Pid(); pid < 1 {
 		t.Errorf("invalid task pid %d", pid)
 	}
 	if err := task.Start(ctx); err != nil {