workqueue: always clear WORKER_REBIND in busy_worker_rebind_fn()

busy_worker_rebind_fn() didn't clear WORKER_REBIND if rebinding failed
(CPU is down again).  This used to be okay because the flag wasn't
used for anything else.

However, after 25511a477 "workqueue: reimplement CPU online rebinding
to handle idle workers", WORKER_REBIND is also used to command idle
workers to rebind.  If not cleared, the worker may confuse the next
CPU_UP cycle by having REBIND spuriously set or oops / get stuck by
prematurely calling idle_worker_rebind().

  WARNING: at /work/os/wq/kernel/workqueue.c:1323 worker_thread+0x4cd/0x5
 00()
  Hardware name: Bochs
  Modules linked in: test_wq(O-)
  Pid: 33, comm: kworker/1:1 Tainted: G           O 3.6.0-rc1-work+ #3
  Call Trace:
   [<ffffffff8109039f>] warn_slowpath_common+0x7f/0xc0
   [<ffffffff810903fa>] warn_slowpath_null+0x1a/0x20
   [<ffffffff810b3f1d>] worker_thread+0x4cd/0x500
   [<ffffffff810bc16e>] kthread+0xbe/0xd0
   [<ffffffff81bd2664>] kernel_thread_helper+0x4/0x10
  ---[ end trace e977cf20f4661968 ]---
  BUG: unable to handle kernel NULL pointer dereference at           (null)
  IP: [<ffffffff810b3db0>] worker_thread+0x360/0x500
  PGD 0
  Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
  Modules linked in: test_wq(O-)
  CPU 0
  Pid: 33, comm: kworker/1:1 Tainted: G        W  O 3.6.0-rc1-work+ #3 Bochs Bochs
  RIP: 0010:[<ffffffff810b3db0>]  [<ffffffff810b3db0>] worker_thread+0x360/0x500
  RSP: 0018:ffff88001e1c9de0  EFLAGS: 00010086
  RAX: 0000000000000000 RBX: ffff88001e633e00 RCX: 0000000000004140
  RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000009
  RBP: ffff88001e1c9ea0 R08: 0000000000000000 R09: 0000000000000001
  R10: 0000000000000002 R11: 0000000000000000 R12: ffff88001fc8d580
  R13: ffff88001fc8d590 R14: ffff88001e633e20 R15: ffff88001e1c6900
  FS:  0000000000000000(0000) GS:ffff88001fc00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
  CR2: 0000000000000000 CR3: 00000000130e8000 CR4: 00000000000006f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
  Process kworker/1:1 (pid: 33, threadinfo ffff88001e1c8000, task ffff88001e1c6900)
  Stack:
   ffff880000000000 ffff88001e1c9e40 0000000000000001 ffff88001e1c8010
   ffff88001e519c78 ffff88001e1c9e58 ffff88001e1c6900 ffff88001e1c6900
   ffff88001e1c6900 ffff88001e1c6900 ffff88001fc8d340 ffff88001fc8d340
  Call Trace:
   [<ffffffff810bc16e>] kthread+0xbe/0xd0
   [<ffffffff81bd2664>] kernel_thread_helper+0x4/0x10
  Code: b1 00 f6 43 48 02 0f 85 91 01 00 00 48 8b 43 38 48 89 df 48 8b 00 48 89 45 90 e8 ac f0 ff ff 3c 01 0f 85 60 01 00 00 48 8b 53 50 <8b> 02 83 e8 01 85 c0 89 02 0f 84 3b 01 00 00 48 8b 43 38 48 8b
  RIP  [<ffffffff810b3db0>] worker_thread+0x360/0x500
   RSP <ffff88001e1c9de0>
  CR2: 0000000000000000

There was no reason to keep WORKER_REBIND on failure in the first
place - WORKER_UNBOUND is guaranteed to be set in such cases
preventing incorrectly activating concurrency management.  Always
clear WORKER_REBIND.

tj: Updated comment and description.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Merge branch 'for-3.6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq

Pull another workqueue fix from Tejun Heo:
 "Unfortunately, yet another late fix.  This too is discovered and fixed
  by Lai.  This bug was introduced during this merge window by commit
  25511a477657 ("workqueue: reimplement CPU online rebinding to handle
  idle workers") which started using WORKER_REBIND flag for idle rebind
  too.

  The bug is relatively easy to trigger if the CPU rapidly goes through
  off, on and then off (and stay off).  The fix is on the safer side.
  This hasn't been on linux-next yet but I'm pushing early so that it
  can get more exposure before v3.6 release."

* 'for-3.6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq:
  workqueue: always clear WORKER_REBIND in busy_worker_rebind_fn()