userspace: add gre sequence number support.

The patch adds support for gre sequence number.
Default is disable.  When enable with 'options:seq=true',
the outgoing gre packet will have its sequence number
incremented by one.

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

lib/netdev.h

@@ -127,6 +127,9 @@ struct netdev_tunnel_config {
     bool csum;
     bool dont_fragment;
     enum netdev_pt_mode pt_mode;
+
+    bool set_seq;
+    uint32_t seqno;
 };
 
 void netdev_run(void);