We should zeroify all data actually for security
reason but this aspect will be addressed in further
patches.

Meanwhile at least allocate enought space so pb_write
won't access data which is not allocated for us.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>