If a temporary mount is a shared one, a new mount can be
propagated into it.

Fixes: 0e9736ab ("mount: fix restoring a bind-mount when its root is overmounted)")

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