userspace: Enable TSO support for non-DPDK.

This patch enables TSO support for non-DPDK use cases, and
also add check-system-tso testsuite. Before TSO, we have to
disable checksum offload, allowing the kernel to calculate the
TCP/UDP packet checsum. With TSO, we can skip the checksum
validation by enabling checksum offload, and with large packet
size, we see better performance.

Consider container to container use cases:
  iperf3 -c (ns0) -> veth peer -> OVS -> veth peer -> iperf3 -s (ns1)
And I got around 6Gbps, similar to TSO with DPDK-enabled.

Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: William Tu <u9012063@gmail.com>

lib/dp-packet.c

@@ -192,10 +192,8 @@ dp_packet_clone_with_headroom(const struct dp_packet *buffer, size_t headroom)
             sizeof(struct dp_packet) -
             offsetof(struct dp_packet, l2_pad_size));
 
-#ifdef DPDK_NETDEV
-    new_buffer->mbuf.ol_flags = buffer->mbuf.ol_flags;
-    new_buffer->mbuf.ol_flags &= ~DPDK_MBUF_NON_OFFLOADING_FLAGS;
-#endif
+    *dp_packet_ol_flags_ptr(new_buffer) = *dp_packet_ol_flags_ptr(buffer);
+    *dp_packet_ol_flags_ptr(new_buffer) &= DP_PACKET_OL_SUPPORTED_MASK;
 
     if (dp_packet_rss_valid(buffer)) {
         dp_packet_set_rss_hash(new_buffer, dp_packet_get_rss_hash(buffer));