prevent rmmod of forced modules

I found a 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 backtrace of
  a task, but the core module ignores it because of the force flag, so
  the unloading succeeds.
- 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 (but still allow them to be
disabled).

An annoying side effect of this approach is that forced modules stay
loaded and in memory forever.  And that after "kpatch unload" of a
forced module, you can't ever load it again because the previous
instance of it is still loaded (but permanently disabled).

This is ugly but I can't really think of a better way to handle it.  If
necessary we could create a workqueue and periodically check to see if
we can safely call module_put() so that the module could be eventually
removed.
code review fix

Return 0 on rmmod error.
Merge pull request #305 from jpoimboe/force-prevent-rmmod

prevent rmmod of forced modules