module: Unlink module with RCU synchronizing instead of stop_machine

Unlink module from module list with RCU synchronizing instead
of using stop_machine(). Since module list is already protected
by rcu, we don't need stop_machine() anymore.

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>