block: use current active bfqq to update statistics

Use the current active bfq-queue to update bfq-group statitics.

It could be possible that the current active serving bfq-queue can
expire if the allocated time/budget for the queue got expired.
During this time, it will select a new queue that to be served
for its service tree.

If there were no more queues to be served, then it will choose a
next group of queues to be served from its group service tree.

So, the selection of the new request from its new group and queue
are updated via __bfq_dispatch_request() fn.

As "in_serv_queue" variable is not updated again the group
associated with "in_serv_queue" queue can be freed, if there were
no more active queues.

So, with picking in_serv_queue as active queue, and updating its group
statistics one will see a kernel panic as below.

[  120.572960] Hardware name: Qualcomm Technologies, Inc. Lito MTP (DT)
[  120.572973] Workqueue: kblockd blk_mq_run_work_fn
[  120.572979] pstate: a0c00085 (NzCv daIf +PAN +UAO)
[  120.572987] pc : bfqg_stats_update_idle_time+0x14/0x50
[  120.572992] lr : bfq_dispatch_request+0x398/0x948

[  121.185249] Call trace:
[  121.187772]  bfqg_stats_update_idle_time+0x14/0x50
[  121.192700]  bfq_dispatch_request+0x398/0x948
[  121.197187]  blk_mq_do_dispatch_sched+0x84/0x118
[  121.198270] CPU7: update max cpu_capacity 1024
[  121.206504]  blk_mq_sched_dispatch_requests+0x130/0x190
[  121.211873]  __blk_mq_run_hw_queue+0xcc/0x148
[  121.216359]  blk_mq_run_work_fn+0x24/0x30
[  121.220489]  process_one_work+0x328/0x6b0
[  121.224619]  worker_thread+0x330/0x4d0
[  121.228475]  kthread+0x128/0x138
[  121.231806]  ret_from_fork+0x10/0x1c

To avoid this, always use the current active bfq-queue, which is
derived from the current active serving request.

Change-Id: I51d5b9d2020da9f3a3a31378b06257463afd08eb
Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>