bdi: Do not use freezable workqueue

A removable block device, such as NVMe or SSD connected over Thunderbolt
can be hot-removed any time including when the system is suspended. When
device is hot-removed during suspend and the system gets resumed, kernel
first resumes devices and then thaws the userspace including freezable
workqueues. What happens in that case is that the NVMe driver notices
that the device is unplugged and removes it from the system. This ends
up calling bdi_unregister() for the gendisk which then schedules
wb_workfn() to be run one more time.

However, since the bdi_wq is still frozen flush_delayed_work() call in
wb_shutdown() blocks forever halting system resume process. User sees
this as hang as nothing is happening anymore.

Triggering sysrq-w reveals this:

  Workqueue: nvme-wq nvme_remove_dead_ctrl_work [nvme]
  Call Trace:
   ? __schedule+0x2c5/0x630
   ? wait_for_completion+0xa4/0x120
   schedule+0x3e/0xc0
   schedule_timeout+0x1c9/0x320
   ? resched_curr+0x1f/0xd0
   ? wait_for...