Commit 85686d0d ("block, bfq: keep shared queues out of the waker
mechanism") leaves shared bfq_queues out of the waker-detection
mechanism. It attains this goal by not updating the pointer
last_completed_rq_bfqq, if the last request completed belongs to a
shared bfq_queue (so that the pointer will not point to the shared
bfq_queue).

Yet this has a side effect: the pointer last_completed_rq_bfqq keeps
pointing, deceptively, to a bfq_queue that actually is not the last
one to have had a request completed. As a consequence, such a
bfq_queue may deceptively be considered as a waker of some bfq_queue,
even of some shared bfq_queue.

To address this issue, reset last_completed_rq_bfqq if the last
request completed belongs to a shared queue.

Fixes: 85686d0d ("block, bfq: keep shared queues out of the waker mechanism")
Signed-off-by: NPaolo Valente <paolo.valente@linaro.org>
Link: https://lore.kernel.org/r/20210619140948.98712-8-paolo.valente@linaro.orgSigned-off-by: NJens Axboe <axboe@kernel.dk>