io_uring: replace defer task_work llist with io_wq_work_list

io_uring: replace defer task_work llist with io_wq_work_list

Add a spinlock for the list, and replace the lockless llist with the
work list instead. This avoids needing to reverse items in the list
before running them, as the io_wq_work_list is FIFO by nature whereas
the llist is LIFO.

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