io_uring: maintain drain logic for multishot poll requests

Now that we have multishot poll requests, one SQE can emit multiple
CQEs. given below example:
    sqe0(multishot poll)-->sqe1-->sqe2(drain req)
sqe2 is designed to issue after sqe0 and sqe1 completed, but since sqe0
is a multishot poll request, sqe2 may be issued after sqe0's event
triggered twice before sqe1 completed. This isn't what users leverage
drain requests for.
Here the solution is to wait for multishot poll requests fully
completed.
To achieve this, we should reconsider the req_need_defer equation, the
original one is:

    all_sqes(excluding dropped ones) == all_cqes(including dropped ones)

This means we issue a drain request when all the previous submitted
SQEs have generated their CQEs.
Now we should consider multishot requests, we deduct all the multishot
CQEs except the cancellation one, In this way a multishot poll request
behave like a normal request, so:
    all_sqes == all_cqes - multishot_cqes(except cancellations)

Here we introduce cq_extra for it.

Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/1618298439-136286-1-git-send-email-haoxu@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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