block: call rq_qos_done() before ref check in batch completions

We need to call rq_qos_done() regardless of whether or not we're freeing
the request or not, as the reference count doesn't cover the IO completion
tracking.

Fixes: f794f335 ("block: add support for blk_mq_end_request_batch()")
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reported-by: Kenneth R. Crudup <kenny@panix.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>