dd6c3c54 ("workqueue: Move pwq_dec_nr_in_flight() to the end of work
item handling") relocated pwq_dec_nr_in_flight() after
set_work_pool_and_keep_pending(). However, the latter destroys information
contained in work->data that's needed by pwq_dec_nr_in_flight() including
the flush color. With flush color destroyed, flush_workqueue() can stall
easily when mixed with cancel_work*() usages.

This is easily triggered by running xfstests generic/001 test on xfs:

     INFO: task umount:6305 blocked for more than 122 seconds.
     ...
     task:umount          state:D stack:13008 pid:6305  tgid:6305  ppid:6301   flags:0x00004000
     Call Trace:
      <TASK>
      __schedule+0x2f6/0xa20
      schedule+0x36/0xb0
      schedule_timeout+0x20b/0x280
      wait_for_completion+0x8a/0x140
      __flush_workqueue+0x11a/0x3b0
      xfs_inodegc_flush+0x24/0xf0
      xfs_unmountfs+0x14/0x180
      xfs_fs_put_super+0x3d/0x90
      generic_shutdown_super+0x7c/0x160
      kill_block_super+0x1b/0x40
      xfs_kill_sb+0x12/0x30
      deactivate_locked_super+0x35/0x90
      deactivate_super+0x42/0x50
      cleanup_mnt+0x109/0x170
      __cleanup_mnt+0x12/0x20
      task_work_run+0x60/0x90
      syscall_exit_to_user_mode+0x146/0x150
      do_syscall_64+0x5d/0x110
      entry_SYSCALL_64_after_hwframe+0x6c/0x74

Fix it by stashing work_data before calling set_work_pool_and_keep_pending()
and using the stashed value for pwq_dec_nr_in_flight().
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Chandan Babu R <chandanbabu@kernel.org>
Link: http://lkml.kernel.org/r/87o7cxeehy.fsf@debian-BULLSEYE-live-builder-AMD64
Fixes: dd6c3c54 ("workqueue: Move pwq_dec_nr_in_flight() to the end of work item handling")