io_uring/sqpoll: use the correct check for pending task_work

A previous commit moved to using just the private task_work list for
SQPOLL, but it neglected to update the check for whether we have
pending task_work. Normally this is fine as we'll attempt to run it
unconditionally, but if we race with going to sleep AND task_work
being added, then we certainly need the right check here.

Fixes: af5d68f8 ("io_uring/sqpoll: manage task_work privately")
Signed-off-by: Jens Axboe <axboe@kernel.dk>