[NETFILTER]: Remove okfn usage in ip_vs_core.c

okfn should only be used from different contexts to avoid deep call chains,
i.e. by nf_queue.

Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>