io_uring: make ASYNC_CANCEL work with poll and timeout

It's a little confusing that we have multiple types of command
cancellation opcodes now that we have a generic one. Make the generic
one work with POLL_ADD and TIMEOUT commands as well, that makes for an
easier to use API for the application. The fact that they currently
don't is a bit confusing.

Add a helper that takes care of it, so we can user it from both
IORING_OP_ASYNC_CANCEL and from the linked timeout cancellation.

Reported-by: Hrvoje Zeba <zeba.hrvoje@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>