mount: restore a shared group for propagated mounts

When a mount is propagated, the result mount is always shared.
If we want to get a private mount, we need to convert it.

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