bridge: fix potential crash in __netdev_pick_tx()

Commit c29390c6 ("xps: must clear sender_cpu before forwarding")
fixed an issue in normal forward path, caused by sender_cpu & napi_id
skb fields being an union.

Bridge is another point where skb can be forwarded, so we need
the same cure.

Bug triggers if packet was received on a NIC using skb_mark_napi_id()

Fixes: 2bd82484

 ("xps: fix xps for stacked devices")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Bob Liu <bob.liu@oracle.com>
Tested-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>