In case if there is no CLONE_NEWCGROUP on root
namespace mask the @private member remains nil
and comparision in mounts_sb_equal leads to nil
dereference.

CC: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>