blk-iocost: fix operation ordering in iocg_wake_fn()

commit 5ab189cf upstream.

iocg_wake_fn() open-codes wait_queue_entry removal and wakeup because it
wants the wq_entry to be always removed whether it ended up waking the
task or not. finish_wait() tests whether wq_entry needs removal without
grabbing the wait_queue lock and expects the waker to use
list_del_init_careful() after all waking operations are complete, which
iocg_wake_fn() didn't do. The operation order was wrong and the regular
list_del_init() was used.

The result is that if a waiter wakes up racing the waker, it can free pop
the wq_entry off stack before the waker is still looking at it, which can
lead to a backtrace like the following.

  [7312084.588951] general protection fault, probably for non-canonical address 0x586bf4005b2b88: 0000 [#1] SMP
  ...
  [7312084.647079] RIP: 0010:queued_spin_lock_slowpath+0x171/0x1b0
  ...
  [7312084.858314] Call Trace:
  [7312084.863548]  _raw_spin_lock_irqsave+0x22/0x30...