net: sched: Fix use after free in red_enqueue()

We can't use "skb" again after passing it to qdisc_enqueue().  This is
basically identical to commit 2f09707d ("sch_sfb: Also store skb
len before calling child enqueue").

Fixes: d7f4f332 ("sch_red: update backlog as well")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>