io-wq: ensure we exit if thread group is exiting

Dave reports that a coredumping workload gets stuck in 5.15-rc2, and
identified the culprit in the Fixes line below. The problem is that
relying solely on fatal_signal_pending() to gate whether to exit or not
fails miserably if a process gets eg SIGILL sent. Don't exclusively
rely on fatal signals, also check if the thread group is exiting.

Fixes: 15e20db2 ("io-wq: only exit on fatal signals")
Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>