[BUG] io_uring: defer logic based on shared data

From: Pavel Begunkov
Date: Fri Oct 25 2019 - 05:55:37 EST

I found 2 problems with __io_sequence_defer().

1. it uses @sq_dropped, but doesn't consider @cq_overflow
2. @sq_dropped and @cq_overflow are write-shared with userspace, so
it can be maliciously changed.

see sent liburing test (test/defer *_hung()), which left an unkillable
process for me

Yours sincerely,
Pavel Begunkov

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