io_uring: fix leaving invalid req->flags

sqe->flags are subset of req flags, so incorrectly copied may span into
in-kernel flags and wreck havoc, e.g. by setting REQ_F_INFLIGHT.

Fixes: 5be9ad1e

 ("io_uring: optimise io_init_req() flags setting")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>