io_uring: don't use retry based buffered reads for non-async bdev

Some block devices, like dm, bubble back -EAGAIN through the completion
handler. We check for this in io_read(), but don't honor it for when
we have copied the iov. Return -EAGAIN for this case before retrying,
to force punt to io-wq.

Fixes: bcf5a06304d6 ("io_uring: support true async buffered reads, if file provides it")
Reported-by: Zorro Lang <[e-mail]>
Tested-by: Zorro Lang <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

fs/io_uring.c

···
 	struct iov_iter __iter, *iter = &__iter;
 	ssize_t io_size, ret, ret2;
 	size_t iov_count;
+	bool no_async;
 
 	if (req->io)
 		iter = &req->io->rw.iter;
···
 		kiocb->ki_flags &= ~IOCB_NOWAIT;
 
 	/* If the file doesn't support async, just async punt */
-	if (force_nonblock && !io_file_supports_async(req->file, READ))
+	no_async = force_nonblock && !io_file_supports_async(req->file, READ);
+	if (no_async)
 		goto copy_iov;
 
 	ret = rw_verify_area(READ, req->file, io_kiocb_ppos(kiocb), iov_count);
···
 		ret = ret2;
 		goto out_free;
 	}
+	if (no_async)
+		return -EAGAIN;
 	/* it's copied and will be cleaned with ->io */
 	iovec = NULL;
 	/* now use our persistent iterator, if we aren't already */