mount: remove root with MS_PRIVATE before pivot_root

Otherwise if the root is mounted with MS_SHARED, pivot_root fails with EINVAL.

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>