cgroup: fix invalid rcu dereference

cgroup: fix invalid rcu dereference

Commit ad676077a2ae ("device_cgroup: convert device_cgroup internally to
policy + exceptions") removed rcu locks which are needed in
task_devcgroup called in this chain:

  devcgroup_inode_mknod OR __devcgroup_inode_permission ->
    __devcgroup_inode_permission ->
      task_devcgroup ->
        task_subsys_state ->
          task_subsys_state_check.

Change the code so that task_devcgroup is safely called with rcu read
lock held.

  ===============================
  [ INFO: suspicious RCU usage. ]
  3.6.0-rc5-next-20120913+ #42 Not tainted
  -------------------------------
  include/linux/cgroup.h:553 suspicious rcu_dereference_check() usage!

  other info that might help us debug this:

  rcu_scheduler_active = 1, debug_locks = 0
  2 locks held by kdevtmpfs/23:
   #0:  (sb_writers){.+.+.+}, at: [<ffffffff8116873f>]
  mnt_want_write+0x1f/0x50
   #1:  (&sb->s_type->i_mutex_key#3/1){+.+.+.}, at: [<ffffffff811558af>]
  kern_path_create+0x7f/0x170

  stack backtrace:
  Pid: 23, comm: kdevtmpfs Not tainted 3.6.0-rc5-next-20120913+ #42
  Call Trace:
    lockdep_rcu_suspicious+0xfd/0x130
    devcgroup_inode_mknod+0x19d/0x240
    vfs_mknod+0x71/0xf0
    handle_create.isra.2+0x72/0x200
    devtmpfsd+0x114/0x140
    ? handle_create.isra.2+0x200/0x200
    kthread+0xd6/0xe0
    kernel_thread_helper+0x4/0x10

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Dave Jones <davej@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: James Morris <jmorris@namei.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>