io-wq: add intermediate work step between pending list and active work

io-wq: add intermediate work step between pending list and active work

We have a gap where a worker removes an item from the work list and to
when it gets added as the workers active work. In this state, the work
item cannot be found by cancelations. This is a small window, but it does
exist.

Add a temporary pointer to a work item that isn't on the pending work
list anymore, but also not the active work. This is needed as we need
to drop the wqe lock in between grabbing the work item and marking it
as active, to ensure that signal based cancelations are properly
ordered.

Reported-by: Florian Fischer <florian.fl.fischer@fau.de>
Link: https://lore.kernel.org/io-uring/20220118151337.fac6cthvbnu7icoc@pasture/
Signed-off-by: Jens Axboe <axboe@kernel.dk>