io_uring/kbuf: don't truncate end buffer for bundles

If buffers have been peeked for a bundle receive, the kernel will
truncate the end buffer, if the available length is shorter than the
buffer itself. This is unnecessary, as applications iterating bundle
receives must always use the minimum size of the buffer length and the
remaining number of bytes in the bundle. The examples in liburing do
that as well, eg examples/proxy.c.

If the kernel does truncate this buffer AND the current transfer fails,
then the buffer will be left with a smaller size than what is otherwise
available.

Just remove the buffer truncation, as it's not necessary in the first
place.

Link: https://lore.kernel.org/io-uring/CAAEr8jbY60noGj1fw_k91UJRBkyiRVoS6=nLhZ7Svwidjn4CAA@mail.gmail.com/
Reported-by: Federico Brasili <federico.brasili@gmail.com>
Cc: stable@vger.kernel.org
Fixes: 35c8711c8f ("io_uring/kbuf: add helpers for getting/peeking multiple buffers")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.c

@@ -305,7 +305,6 @@ static int io_ring_buffers_peek(struct io_kiocb *req, struct buf_sel_arg *arg,
 				arg->partial_map = 1;
 				if (iov != arg->iovs)
 					break;
-				WRITE_ONCE(buf->len, len);
 			}
 		}