io_uring: don't read user-shared sqe flags twice

Don't re-read userspace-shared sqe->flags, it can be exploited.
sqe->flags are copied into req->flags in io_submit_sqe(), check them
there instead.

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