aio: Fix fallback I/O priority value

For cases when the application does not specify aio_reqprio for an aio,
fallback to use get_current_ioprio() to obtain the task I/O priority
last set using ioprio_set() rather than the hardcoded IOPRIO_CLASS_NONE
value.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/aio.c

@@ -1441,7 +1441,7 @@ static int aio_prep_rw(struct kiocb *req, struct iocb *iocb)
 
 		req->ki_ioprio = iocb->aio_reqprio;
 	} else
-		req->ki_ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, 0);
+		req->ki_ioprio = get_current_ioprio();
 
 	ret = kiocb_set_rw_flags(req, iocb->aio_rw_flags);
 	if (unlikely(ret))