wireguard: device: clear keys on VM fork

When a virtual machine forks, it's important that WireGuard clear
existing sessions so that different plaintexts are not transmitted using
the same key+nonce, which can result in catastrophic cryptographic
failure. To accomplish this, we simply hook into the newly added vmfork
notifier.

As a bonus, it turns out that, like the vmfork registration function,
the PM registration function is stubbed out when CONFIG_PM_SLEEP is not
set, so we can actually just remove the maze of ifdefs, which makes it
really quite clean to support both notifiers at once.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>