netem: refine early skb orphaning

netem does an early orphaning of skbs. Doing so breaks TCP Small Queue
or any mechanism relying on socket sk_wmem_alloc feedback.

Ideally, we should perform this orphaning after the rate module and
before the delay module, to mimic what happens on a real link :

skb orphaning is indeed normally done at TX completion, before the
transit on the link.

+-------+   +--------+  +---------------+  +-----------------+
+ Qdisc +---> Device +--> TX completion +--> links / hops    +->
+       +   +  xmit  +  + skb orphaning +  + propagation     +
+-------+   +--------+  +---------------+  +-----------------+
      < rate limiting >                  < delay, drops, reorders >

If netem is used without delay feature (drops, reorders, rate
limiting), then we should avoid early skb orphaning, to keep pressure
on sockets as long as packets are still in qdisc queue.

Ideally, netem should be refactored to implement delay module
as the last stage. Current algorithm merges the two phases
(rate limiting + delay) so its not correct.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Hagen Paul Pfeifer <hagen@jauu.net>
Cc: Mark Gordon <msg@google.com>
Cc: Andreas Terzis <aterzis@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>