mount: don't use a shared mount to get a clean mount

When we bind-mount something into a shared mount,
a new mount will be shared too.

By default /tmp is a shared mount, so when we get a "clean mount",
it will be always shared. On restore we don't need this side effect,
so let's do bind-mount in a private mount.

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