io_uring: use labeled array init in io_op_defs

Don't rely on implicit ordering of IORING_OP_ and explicitly place them
at a right place in io_op_defs. Now former comments are now a part of
the code and won't ever outdate.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>