block: fix use after free in __blkdev_direct_IO

We can't dereference the dio structure after submitting the last bio for
this request, as I/O completion might have happened before the code is
run. Introduce a local is_sync variable instead.

Fixes: 542ff7bf ("block: new direct I/O implementation")
Signed-off-by: Christoph Hellwig <[e-mail]>
Reported-by: Matias Bjørling <[e-mail]>
Tested-by: Matias Bjørling <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

fs/block_dev.c

···
 	struct blk_plug plug;
 	struct blkdev_dio *dio;
 	struct bio *bio;
-	bool is_read = (iov_iter_rw(iter) == READ);
+	bool is_read = (iov_iter_rw(iter) == READ), is_sync;
 	loff_t pos = iocb->ki_pos;
 	blk_qc_t qc = BLK_QC_T_NONE;
 	int ret;
···
 	bio_get(bio); /* extra ref for the completion handler */
 
 	dio = container_of(bio, struct blkdev_dio, bio);
-	dio->is_sync = is_sync_kiocb(iocb);
+	dio->is_sync = is_sync = is_sync_kiocb(iocb);
 	if (dio->is_sync)
 		dio->waiter = current;
 	else
···
 	}
 	blk_finish_plug(&plug);
 
-	if (!dio->is_sync)
+	if (!is_sync)
 		return -EIOCBQUEUED;
 
 	for (;;) {