io_uring: simplify overflow handling

Overflowed CQEs doesn't lock requests anymore, so we don't care so much
about cancelling them, so kill cq_overflow_flushed and simplify the
code.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5799867aeba9e713c32f49aef78e5e1aef9fbc43.1617287883.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>