integration-cli: fix netns test cleanup

1. Using MNT_FORCE flag does not make sense for nsfs. Using MNT_DETACH
though might help.

2. When -check.vv is added to TESTFLAGS, there are a lot of messages
like this one:

> unmount of /tmp/dxr/d847fd103a4ba/netns failed: invalid argument

and some like

> unmount of /tmp/dxr/dd245af642d94/netns failed: no such file or directory

The first one means directory is not a mount point, the second one
means it's gone. Do ignore both of these.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

internal/test/daemon/daemon_unix.go

@@ -21,7 +21,7 @@ func cleanupNetworkNamespace(t testingT, execRoot string) {
 	// new exec root.
 	netnsPath := filepath.Join(execRoot, "netns")
 	filepath.Walk(netnsPath, func(path string, info os.FileInfo, err error) error {
-		if err := unix.Unmount(path, unix.MNT_FORCE); err != nil {
+		if err := unix.Unmount(path, unix.MNT_DETACH); err != nil && err != unix.EINVAL && err != unix.ENOENT {
 			t.Logf("unmount of %s failed: %v", path, err)
 		}
 		os.Remove(path)