io_uring/kbuf: return correct iovec count from classic buffer peek

io_provided_buffers_select() returns 0 to indicate success, but it should
be returning 1 to indicate that 1 vec was mapped. This causes peeking
to fail with classic provided buffers, and while that's not a use case
that anyone should use, it should still work correctly.

The end result is that no buffer will be selected, and hence a completion
with '0' as the result will be posted, without a buffer attached.

Fixes: 35c8711c ("io_uring/kbuf: add helpers for getting/peeking multiple buffers")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.c

@@ -129,7 +129,7 @@ static int io_provided_buffers_select(struct io_kiocb *req, size_t *len,
 
 	iov[0].iov_base = buf;
 	iov[0].iov_len = *len;
-	return 0;
+	return 1;
 }
 
 static struct io_uring_buf *io_ring_head_to_buf(struct io_uring_buf_ring *br,