io_uring/notif: refactor io_tx_ubuf_complete()

Flip the dec_and_test "if", that makes the function extension easier in
the future.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/43939e2b04dff03bff5d7227c98afedf951227b3.1713185320.git.asml.silence@gmail.com


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