Use mount.Target to specify subdirectory of rootfs mount

- Add Target to mount.Mount.
- Add UnmountMounts to unmount a list of mounts in reverse order.
- Add UnmountRecursive to unmount deepest mount first for a given target, using
moby/sys/mountinfo.

Signed-off-by: Edgar Lee <edgarhinshunlee@gmail.com>

pkg/process/init.go

@@ -307,7 +307,7 @@ func (p *Init) delete(ctx context.Context) error {
 		}
 		p.io.Close()
 	}
-	if err2 := mount.UnmountAll(p.Rootfs, 0); err2 != nil {
+	if err2 := mount.UnmountRecursive(p.Rootfs, 0); err2 != nil {
 		log.G(ctx).WithError(err2).Warn("failed to cleanup rootfs mount")
 		if err == nil {
 			err = fmt.Errorf("failed rootfs umount: %w", err2)