There are two deadlock scenarios that need addressing, which cause
problems when the computer goes to sleep, the interface is set down, and
hwrng_unregister() is called. When the deadlock is hit, sleep is delayed
for tens of seconds, causing it to fail. These scenarios are:

1) The hwrng kthread can't be stopped while it's sleeping, because it
   uses msleep_interruptible() which does not react to kthread_stop.

2) A normal user thread can't be interrupted by hwrng_unregister() while
   it's sleeping, because hwrng_unregister() is called from elsewhere.

We solve both issues by add a completion object called dying that
fulfils waiters once we have started the process in hwrng_unregister.

At the same time, we should cleanup a common and useless dmesg splat
in the same area.

Cc: <stable@vger.kernel.org>
Reported-by: Gregory Erwin <gregerwin256@gmail.com>
Fixes: fcd09c90 ("ath9k: use hw_random API instead of directly dumping into random.c")
Link: https://lore.kernel.org/all/CAO+Okf6ZJC5-nTE_EJUGQtd8JiCkiEHytGgDsFGTEjs0c00giw@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAO+Okf5k+C+SE6pMVfPf-d8MfVPVq4PO7EY8Hys_DVXtent3HA@mail.gmail.com/
Link: https://bugs.archlinux.org/task/75138Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Acked-by: Kalle Valo <kvalo@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>