Revert "blk-rq-qos: remove redundant finish_wait to rq_qos_wait."

This reverts commit 826f2f48.

Qian Cai reports that this commit causes stalls with swap. Revert until
the reason can be figured out.

Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Jens Axboe <axboe@kernel.dk>