cpuset: fix RCU lockdep splat

cpuset: fix RCU lockdep splat

5d21cc2db040d01f8c19b8602f6987813e1176b4 ("cpuset: replace
cgroup_mutex locking with cpuset internal locking") incorrectly
converted proc_cpuset_show() from cgroup_lock() to cpuset_mutex.
proc_cpuset_show() is accessing cgroup hierarchy proper to determine
cgroup path which can't be protected by cpuset_mutex.  This triggered
the following RCU warning.

 ===============================
 [ INFO: suspicious RCU usage. ]
 3.8.0-rc3-next-20130114-sasha-00016-ga107525-dirty #262 Tainted: G        W
 -------------------------------
 include/linux/cgroup.h:534 suspicious rcu_dereference_check() usage!

 other info that might help us debug this:

 rcu_scheduler_active = 1, debug_locks = 1
 2 locks held by trinity/7514:
  #0:  (&p->lock){+.+.+.}, at: [<ffffffff812b06aa>] seq_read+0x3a/0x3e0
  #1:  (cpuset_mutex){+.+...}, at: [<ffffffff811abae4>] proc_cpuset_show+0x84/0x190

 stack backtrace:
 Pid: 7514, comm: trinity Tainted: G        W
+3.8.0-rc3-next-20130114-sasha-00016-ga107525-dirty #262
 Call Trace:
  [<ffffffff81182cab>] lockdep_rcu_suspicious+0x10b/0x120
  [<ffffffff811abb71>] proc_cpuset_show+0x111/0x190
  [<ffffffff812b0827>] seq_read+0x1b7/0x3e0
  [<ffffffff812b0670>] ? seq_lseek+0x110/0x110
  [<ffffffff8128b4fb>] do_loop_readv_writev+0x4b/0x90
  [<ffffffff8128b776>] do_readv_writev+0xf6/0x1d0
  [<ffffffff8128b8ee>] vfs_readv+0x3e/0x60
  [<ffffffff8128b960>] sys_readv+0x50/0xd0
  [<ffffffff83d33d18>] tracesys+0xe1/0xe6

The operation can be performed under RCU read lock.  Replace
cpuset_mutex locking with RCU read locking.

tj: Rewrote patch description.

Reported-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>