net: add wait to iptables command to fix race

when run several tests simultaneousely it seem that some iptables
commands can intersect and if we do not wait xtables lock criu fails:

(00.009263)     Running iptables [iptables -t filter -A INPUT --protocol tcp --source 127.0.0.1 --sport 55074 --destination 127.0.0.1 --dport 8880 -j DROP]
Another app is currently holding the xtables lock. Perhaps you want to use the -w option?
(00.014367) Error (util.c:660): exited, status=4
(00.014416) Error (netfilter.c:88): Iptables configuration failed: Success
(00.014432) ----------------------------------------
(00.014461) Error (cr-dump.c:1297): Dump files (pid: 24) failed with -1

https://ci.openvz.org/job/CRIU/job/CRIU-virtuozzo/branch/criu-dev/3/



Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>