cgroup: net_cls: Remove rcu_read_lock/unlock

As Eric pointed out:
"Hey task_cls_classid() has its own rcu protection since commit
3fb5a991 (cls_cgroup: Fix rcu lockdep warning)

So we can safely revert Paul commit (1144182a)
(We no longer need rcu_read_lock/unlock here)"

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: netdev@vger.kernel.org
Cc: cgroups@vger.kernel.org
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>