io_uring: ensure IOPOLL locks around deferred work

No direct upstream commit exists for this issue. It was fixed in
5.18 as part of a larger rework of the completion side.

io_commit_cqring() writes the CQ ring tail to make it visible, but it
also kicks off any deferred work we have. A ring setup with IOPOLL
does not need any locking around the CQ ring updates, as we're always
under the ctx uring_lock. But if we have deferred work that needs
processing, then io_queue_deferred() assumes that the completion_lock
is held, as it is for !IOPOLL.

Add a lockdep assertion to check and document this fact, and have
io_iopoll_complete() check if we have deferred work and run that
separately with the appropriate lock grabbed.

Cc: stable@vger.kernel.org # 5.10, 5.15
Reported-by: dghost david <daviduniverse18@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>