netfilter: add a xt_FLOWOFFLOAD target for NAT/routing offload support

This makes it possible to add an iptables rule that offloads routing/NAT
packet processing to a software fast path. This fast path is much
quicker than running packets through the regular tables/chains.

Requires Linux 4.14

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/network/utils/iptables/patches/800-flowoffload_target.patch

@@ -0,0 +1,18 @@
+--- /dev/null
++++ b/extensions/libxt_FLOWOFFLOAD.c
+@@ -0,0 +1,15 @@
++#include <xtables.h>
++
++static struct xtables_target offload_tg_reg[] = {
++	{
++		.family        = NFPROTO_UNSPEC,
++		.name          = "FLOWOFFLOAD",
++		.revision      = 0,
++		.version       = XTABLES_VERSION,
++	},
++};
++
++void _init(void)
++{
++	xtables_register_targets(offload_tg_reg, ARRAY_SIZE(offload_tg_reg));
++}