blk_mq_tagset_count_completed_reqs returns the number of completed
requests. The only user of this function is
blk_mq_tagset_wait_completed_request which wants to know how many
request are not yet completed. Thus return the number of in flight
requests and terminate the wait loop when there is no inflight request.
Fixes: f9934a80f91d ("blk-mq: introduce blk_mq_tagset_wait_completed_request()")