ovs-vswitchd: Avoid segfault for "netdev" datapath.

ovs-vswitchd: Avoid segfault for "netdev" datapath.

When the datapath, whose type is "netdev", processes packets
in userspce action, it may cause a segmentation fault. In the
dp_execute_userspace_action(), we pass the "wc" argument to
dp_netdev_upcall() using NULL. In the dp_netdev_upcall() call tree,
the "wc" will be used. For example, dp_netdev_upcall() uses the
&wc->masks for debugging, and flow_wildcards_init_for_packet()
uses the  "wc" if we disable megaflow, which is described in
more detail below.

Segmentation fault in flow_wildcards_init_for_packet:

    #0  0x0000000000468fe8 flow_wildcards_init_for_packet lib/flow.c:1275
    #1  0x0000000000436c0b upcall_cb ofproto/ofproto-dpif-upcall.c:1231
    #2  0x000000000045bd96 dp_netdev_upcall lib/dpif-netdev.c:3857
    #3  0x0000000000461bf3 dp_execute_userspace_action lib/dpif-netdev.c:4388
    #4  dp_execute_cb lib/dpif-netdev.c:4521
    #5  0x0000000000486ae2 odp_execute_actions lib/odp-execute.c:538
    #6  0x00000000004607f9 dp_netdev_execute_actions lib/dpif-netdev.c:4627
    #7  packet_batch_per_flow_execute lib/dpif-netdev.c:3927
    #8  dp_netdev_input__ lib/dpif-netdev.c:4229
    #9  0x0000000000460ba8 dp_netdev_input lib/dpif-netdev.c:4238
    #10 dp_netdev_process_rxq_port lib/dpif-netdev.c:2873
    #11 0x000000000046126e dpif_netdev_run lib/dpif-netdev.c:3000
    #12 0x000000000042baf5 type_run ofproto/ofproto-dpif.c:504
    #13 0x00000000004192bf ofproto_type_run ofproto/ofproto.c:1687
    #14 0x0000000000409965 bridge_run__ vswitchd/bridge.c:2875
    #15 0x000000000040f145 bridge_run vswitchd/bridge.c:2938
    #16 0x00000000004062e5 main vswitchd/ovs-vswitchd.c:111

Signed-off-by: nickcooper-zhangtonghao <nic@opencloud.tech>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>