io_uring/io-wq: don't clear PF_IO_WORKER on exit

A recent commit gated the core dumping task exit logic on current->flags
remaining consistent in terms of PF_{IO,USER}_WORKER at task exit time.
This exposed a problem with the io-wq handling of that, which explicitly
clears PF_IO_WORKER before calling do_exit().

The reasons for this manual clear of PF_IO_WORKER is historical, where
io-wq used to potentially trigger a sleep on exit. As the io-wq thread
is exiting, it should not participate any further accounting. But these
days we don't need to rely on current->flags anymore, so we can safely
remove the PF_IO_WORKER clearing.

Reported-by: Zorro Lang <zlang@redhat.com>
Reported-by: Dave Chinner <david@fromorbit.com>
Link: https://lore.kernel.org/all/ZIZSPyzReZkGBEFy@dread.disaster.area/
Fixes: f9010dbd

 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>