io_uring/waitid: fix KCSAN warning on io_waitid->head

io_uring/waitid: fix KCSAN warning on io_waitid->head

Storing of the iw->head entry inside the wait_queue callback, or when
removing a waitid item, really should use proper load/store
acquire/release semantics, and KCSAN correctly warns of that. Ensure
that they do so.

Reported-by: syzbot+eb441775f4f948a0902f@syzkaller.appspotmail.com
Fixes: a48c0cbf28c0 ("io_uring/waitid: have io_waitid_complete() remove wait queue entry")
Signed-off-by: Jens Axboe <axboe@kernel.dk>