io_uring: make reg buf init consistent

The default (i.e. empty) state of register buffer is dummy_ubuf, so set
it to dummy on init instead of NULL.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c5456aecf03d9627fbd6e65e100e2b5293a6151e.1655310733.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>