net_sched: fix suspicious RCU usage in tcindex_classify()

This patch fixes the following kernel warning:

[   44.805900] [ INFO: suspicious RCU usage. ]
[   44.808946] 3.17.0-rc4+ #610 Not tainted
[   44.811831] -------------------------------
[   44.814873] net/sched/cls_tcindex.c:84 suspicious rcu_dereference_check() usage!

Fixes: commit 331b7292

 ("net: sched: RCU cls_tcindex")
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>