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.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>