block: exit_io_context() should call elevator_exit_icq_fn()

While updating locking, b2efa052 "block, cfq: unlink
cfq_io_context's immediately" moved elevator_exit_icq_fn() invocation
from exit_io_context() to the final ioc put.  While this doesn't cause
catastrophic failure, it effectively removes task exit notification to
elevator and cause noticeable IO performance degradation with CFQ.

On task exit, CFQ used to immediately expire the slice if it was being
used by the exiting task as no more IO would be issued by the task;
however, after b2efa052, the notification is lost and disk could sit
idle needlessly, leading to noticeable IO performance degradation for
certain workloads.

This patch renames ioc_exit_icq() to ioc_destroy_icq(), separates
elevator_exit_icq_fn() invocation into ioc_exit_icq() and invokes it
from exit_io_context().  ICQ_EXITED flag is added to avoid invoking
the callback more than once for the same icq.

Walking icq_list from ioc side and invoking elevator callback requires
reverse double locking.  This may be better implemented using RCU;
unfortunately, using RCU isn't trivial.  e.g. RCU protection would
need to cover request_queue and queue_lock switch on cleanup makes
grabbing queue_lock from RCU unsafe.  Reverse double locking should
do, at least for now.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-bisected-by: Shaohua Li <shli@kernel.org>
LKML-Reference: <CANejiEVzs=pUhQSTvUppkDcc2TNZyfohBRLygW5zFmXyk5A-xQ@mail.gmail.com>
Tested-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>