io_uring: kill io_cq_unlock()

We're abusing ->completion_lock helpers. io_cq_unlock() neither
locking conditionally nor doing CQE flushing, which means that callers
must have some side reason of taking the lock and should do it directly.

Open code io_cq_unlock() into io_cqring_overflow_kill() and clean it up.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7dabb36856db2b562e78780480396c52c29b2bf4.1687518903.git.asml.silence@gmail.com


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