io-wq: exclusively gate signal based exit on get_signal() return

io-wq threads block all signals, except SIGKILL and SIGSTOP. We should not
need any extra checking of signal_pending or fatal_signal_pending, rely
exclusively on whether or not get_signal() tells us to exit.

The original debugging of this issue led to the false positive that we
were exiting on non-fatal signals, but that is not the case. The issue
was around races with nr_workers accounting.

Fixes: 87c16966 ("io-wq: ensure we exit if thread group is exiting")
Fixes: 15e20db2 ("io-wq: only exit on fatal signals")
Reported-by: Eric W. Biederman <ebiederm@xmission.com>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>