From: Rusty Russell <rusty@rustcorp.com.au>

Sam Ravnborg found these.

1) have_vmlinux is a global, and should not be reset every time.

2) We pretend every module needs cleanup_module so it gets versioned,
   but that isn't defined for CONFIG_MODULE_UNLOAD=n.

3) The visible effect of this is that modpost will start complaning about
   undefined symbols - previously this happened only when the module was
   isntalled.