For placeholder buffers, &dummy_ubuf is assigned which is a static
value. When buffers are attempted cloned, don't attempt to grab a
reference to it, as we both don't need it and it'll actively fail as
dummy_ubuf doesn't have a valid reference count setup.

Link: https://lore.kernel.org/io-uring/Zw8dkUzsxQ5LgAJL@ly-workstation/Reported-by: Lai, Yi <yi1.lai@linux.intel.com>
Fixes: 7cc2a6ea ("io_uring: add IORING_REGISTER_COPY_BUFFERS method")
Signed-off-by: Jens Axboe <axboe@kernel.dk>