restore: Defer new net-namespace creation until forked

Some kernel modules such as pktgen runs kthred upon
new-net creation taking last_pid we were requested.
Lets workaround this problem using clone + unshare
bundle.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>