io_uring: switch deferred task_work to an io_wq_work_list

io_uring: switch deferred task_work to an io_wq_work_list

Lockless lists may be handy for some things, but they mean that items
are in the reverse order as we can only add to the head of the list.
That in turn means that iterating items on the list needs to reverse it
first, if it's sensitive to ordering between items on the list.

Switch the DEFER_TASKRUN work list from an llist to a normal
io_wq_work_list, and protect it with a lock. Then we can get rid of the
manual reversing of the list when running it, which takes considerable
cycles particularly for bursty task_work additions.

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