io_uring/sqpoll: annotate debug task == current with data_race()

There's a debug check in io_sq_thread_park() checking if it's the SQPOLL
thread itself calling park. KCSAN warns about this, as we should not be
reading sqd->thread outside of sqd->lock.

Just silence this with data_race(). The pointer isn't used for anything
but this debug check.

Reported-by:  <syzbot+2b946a3fd80caf971b21@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>