dm-delay: fix bugs introduced by kthread mode

This commit fixes the following bugs introduced by commit 70bbeb29fab0
("dm delay: for short delays, use kthread instead of timers and wq"):

* the function flush_worker_fn has no exit path - on unload, this
  function will just loop and consume 100% CPU without any progress

* the wake-up mechanism in flush_worker_fn is racy - a wake up will be
  missed if the process adds entries to the delayed_bios list just
  before set_current_state(TASK_INTERRUPTIBLE)

* flush_delayed_bios_fast submits a bio while holding a global mutex;
  this may deadlock if we have multiple stacked dm-delay devices and
  the underlying device attempts to acquire the mutex too

* if the target constructor fails, it will call delay_dtr. delay_dtr
  would attempt to free dc->timer_lock without it being initialized by
  the constructor.

* if the target constructor's kthread allocation fails, delay_dtr
  would crash trying to dereference dc->worker because it is non-NULL
  due to ERR_PTR.

Fixes: 70bbeb29fab0 ("dm delay: for short delays, use kthread instead of timers and wq")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

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