Here is a race when someone umounted something and this operation
isn't propagated into our namespace.

CRIU                                | Another process
-----------------------------------------------------------------
pivot_root(".", put_root)           |
mount(put_root, MS_REC|MS_PRIVATE)  |
                                    | umount /xxx/yyy
                                    | umount /xxx -> EBUSY
umount(put_root)

We do this to not affect mounts in put_root, but we can mask
these mounts as slave and this will work for us and for external
users.

Reported-by: Mr Travis
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>