prevent rmmod of forced modules

I found a pretty bad bug:

- Module A is loaded, and registers function foo() with
  KPATCH_FORCE_UNSAFE.
- Module A is unloaded.  The new version of foo() is on the backtrack of
  a task, but the core module ignores it because of the force flag, so
  the unloading is successful.
- The task returns to the new version of foo() which no longer exists.
- BOOM.

The only way I can think of to prevent this scenario is to prevent
forced modules from being unloaded.  It's not ideal, but I don't know of
any better way to handle it...
Merge branch 'force-prevent-rmmod' into for-master