datapath: simplify sample action implementation

datapath: simplify sample action implementation

The current sample() function implementation is more complicated
than necessary in handling single user space action optimization
and skb reference counting. There is no functional changes.

Msg from Chris Dunlop:

The commit isn't actually designed to address the problem directly,
however in simplifying sample() it removes a call to skb_get(). The
skb_get() makes the skb shared, which later causes us to hit the BUG().
E.g. your v2.3.1 stack trace shows this call path:

  netdev_frame_hook
  + netdev_port_receive
    | skb is guaranteed not-shared, via:
    |   skb = skb_share_check(skb, GFP_ATOMIC);
    + ovs_vport_receive
      + ovs_dp_process_received_packet
        + ovs_dp_process_packet_with_key
          + ovs_execute_actions
            + do_execute_actions
              | nla_type(a) == OVS_ACTION_ATTR_SAMPLE
              + sample
                | skb is made shared here, via:
                |   sample_skb = skb;
                |   skb_get(skb);
                + do_execute_actions
                  | nla_type(a) == OVS_ACTION_ATTR_USERSPACE
                  + output_userspace
                    + ovs_dp_upcall
                      + queue_userspace_packet
                        + skb_checksum_help
                          + pskb_expand_head
                            | if (skb_shared(skb))
                            |         BUG();        BOOM!!!

Reported-by: Chris Dunlop <chris@onthe.net.au>
Reported-by: "Xu (Simon) Chen" <xchenum@gmail.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>