io_uring: add missing finish_wait() in io_sq_thread()

Add it to pair with prepare_to_wait() in an attempt to avoid
anything weird in the field.

Fixes: b41e9852 ("io_uring: add per-task callback handler")
Reported-by:  <syzbot+0c3370f235b74b3cfd97@syzkaller.appspotmail.com>
Signed-off-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>