io_uring/notif: simplify io_notif_flush()

io_notif_flush() is partially duplicating io_tx_ubuf_complete(), so
instead of duplicating it, make the flush call io_tx_ubuf_complete.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/19e41652c16718b946a5c80d2ad409df7682e47e.1713369317.git.asml.silence@gmail.com


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