block: don't revert iter for -EIOCBQUEUED

blkdev_read_iter() has a few odd checks, like gating the position and
count adjustment on whether or not the result is bigger-than-or-equal to
zero (where bigger than makes more sense), and not checking the return
value of blkdev_direct_IO() before doing an iov_iter_revert(). The
latter can lead to attempting to revert with a negative value, which
when passed to iov_iter_revert() as an unsigned value will lead to
throwing a WARN_ON() because unroll is bigger than MAX_RW_COUNT.

Be sane and don't revert for -EIOCBQUEUED, like what is done in other
spots.

Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/fops.c

@@ -783,11 +783,12 @@ static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 		file_accessed(iocb->ki_filp);
 
 		ret = blkdev_direct_IO(iocb, to);
-		if (ret >= 0) {
+		if (ret > 0) {
 			iocb->ki_pos += ret;
 			count -= ret;
 		}
-		iov_iter_revert(to, count - iov_iter_count(to));
+		if (ret != -EIOCBQUEUED)
+			iov_iter_revert(to, count - iov_iter_count(to));
 		if (ret < 0 || !count)
 			goto reexpand;
 	}