io_uring: rename "copy buffers" to "clone buffers"

A recent commit added support for copying registered buffers from one
ring to another. But that term is a bit confusing, as no copying of
buffer data is done here. What is being done is simply cloning the
buffer registrations from one ring to another.

Rename it while we still can, so that it's more descriptive. No
functional changes in this patch.

Fixes: 7cc2a6ea ("io_uring: add IORING_REGISTER_COPY_BUFFERS method")
Signed-off-by: Jens Axboe <axboe@kernel.dk>