slot_dequeue_head() should make sure slot skb chain is correct in both
ways, or we can crash if all possible flows are in use.

Jarek pointed out slot_queue_init() can now be done in sfq_init() once,
instead each time a flow is setup.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>