net_sched: sch_netem: defer skb freeing

rtnl_kfree_skbs() can be used in tfifo_reset()

It would be nice if we could iterate through rb tree instead
of removing one skb at a time, and build a single skb chain.
But this is left for a future patch.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>