io_uring: don't repeat valid flag list

req->flags stores all sqe->flags. After checking that sqe->flags are
valid set if IOSQE* flags, no need to double check it, just forward them
all.

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