io_uring: allow IO_SQE_* flags on IORING_OP_TIMEOUT

There's really no reason why we forbid things like link/drain etc on
regular timeout commands. Enable the usual SQE flags on timeouts.

Reported-by: 李通洲 <carter.li@eoitek.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>