restore: add a synchronisation point after restoring credentials

For security reason processes can be resumed only when all
credentials are restored. Otherwise someone can attach to a
process, which are not restored credentials yet and execute
some code.

https://bugzilla.openvz.org/show_bug.cgi?id=2561



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