kernel-netlink: Enable UDP GRO

This enables GRO offload for inbound ESP-in-UDP packets if the
esp4|6_offload modules are loaded.  Note that inbound ESP or ESP-in-UDP
packets won't be visible on layer 3 in Netfilter or tcpdump.

src/libcharon/plugins/kernel_netlink/kernel_netlink_ipsec.c

@@ -3884,6 +3884,12 @@ METHOD(kernel_ipsec_t, enable_udp_decap, bool,
 		DBG1(DBG_KNL, "unable to set UDP_ENCAP: %s", strerror(errno));
 		return FALSE;
 	}
+	type = 1;
+	if (setsockopt(fd, SOL_UDP, UDP_GRO, &type, sizeof(type)) < 0)
+	{
+		DBG1(DBG_KNL, "unable to set UDP_GRO: %s", strerror(errno));
+		return FALSE;
+	}
 	return TRUE;
 }