finish_writeback_work() reads @done->waitq after decrementing
@done->cnt.  However, once @done->cnt reaches zero, @done may be freed
(from stack) at any moment and @done->waitq can contain something
unrelated by the time finish_writeback_work() tries to read it.  This
led to the following crash.

  "BUG: kernel NULL pointer dereference, address: 0000000000000002"
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP DEBUG_PAGEALLOC
  CPU: 40 PID: 555153 Comm: kworker/u98:50 Kdump: loaded Not tainted
  ...
  Workqueue: writeback wb_workfn (flush-btrfs-1)
  RIP: 0010:_raw_spin_lock_irqsave+0x10/0x30
  Code: 48 89 d8 5b c3 e8 50 db 6b ff eb f4 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 53 9c 5b fa 31 c0 ba 01 00 00 00 <f0> 0f b1 17 75 05 48 89 d8 5b c3 89 c6 e8 fe ca 6b ff eb f2 66 90
  RSP: 0018:ffffc90049b27d98 EFLAGS: 00010046
  RAX: 0000000000000000 RBX: 0000000000000246 RCX: 0000000000000000
  RDX: 0000000000000001 RSI: 0000000000000003 RDI: 0000000000000002
  RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
  R10: ffff889fff407600 R11: ffff88ba9395d740 R12: 000000000000e300
  R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000
  FS:  0000000000000000(0000) GS:ffff88bfdfa00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000002 CR3: 0000000002409005 CR4: 00000000001606e0
  Call Trace:
   __wake_up_common_lock+0x63/0xc0
   wb_workfn+0xd2/0x3e0
   process_one_work+0x1f5/0x3f0
   worker_thread+0x2d/0x3d0
   kthread+0x111/0x130
   ret_from_fork+0x1f/0x30

Fix it by reading and caching @done->waitq before decrementing
@done->cnt.

Link: http://lkml.kernel.org/r/20190924010631.GH2233839@devbig004.ftw2.facebook.com
Fixes: 5b9cce4c ("writeback: Generalize and expose wb_completion")
Signed-off-by: Tejun Heo <tj@kernel.org>
Debugged-by: Chris Mason <clm@fb.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: <stable@vger.kernel.org>	[5.2+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>