io_uring: introduce llist helpers

Introduce helpers to atomically switch llist.

Will later move this into common code

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220622134028.2013417-5-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>