Allow IORING_OP_ASYNC_CANCEL to cancel requests on other rings

[Artyom Pavlov <newpavlov@gmail.com>]

Greetings!

Right now when I want to cancel request which runs on a different ring I 
have to use IORING_OP_MSG_RING with a special len value. CQEs with res 
equal to this special value get intercepted by my code and 
IORING_OP_ASYNC_CANCEL SQE gets created in the receiver ring with 
user_data taken from the received message. This approach kind of works, 
but not efficient (it requires additional round trip through the ring) 
and somewhat fragile (it relies on lack of collisions between the 
special value and potential error codes).

I think it should be possible to add support for cancelling requests on 
other rings to IORING_OP_ASYNC_CANCEL by introducing a new flag. If the 
flag is enabled, then the fd field would be interpreted as fd of another 
ring to which cancellation request should be sent. Using the fd field 
would mean that the new flag would conflict with IORING_ASYNC_CANCEL_FD, 
so it could be worth to use a different field for receiver ring fd.

Best regards,
Artyom Pavlov.