io_uring: check IOPOLL/ioprio support upfront

Don't punt this check to the op prep handlers, add the support to
io_op_defs and we can check them while setting up the request.

This reduces the text size by 500 bytes on aarch64, and makes this less
fragile by having the check in one spot and needing opcodes to opt in
to IOPOLL or ioprio support.

Reviewed-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>