module: convert to stop_machine_create/destroy.

The module code relies on a non-failing stop_machine call. So we create
the kstop threads in advance and with that make sure the call won't fail.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>