io_uring: always allow drain/link/hardlink/async sqe flags

We currently filter these for timeout_remove/async_cancel/files_update,
but we only should be filtering for fixed file and buffer select. This
also causes a second read of sqe->flags, which isn't needed.

Just check req->flags for the relevant bits. This then allows these
commands to be used in links, for example, like everything else.

Signed-off-by: Daniele Albano <d.albano@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>