modprobe: fix error path in removing modules

modprobe: fix error path in removing modules

We really haven't paid this code much attention, and it's somewhat
evident in our divergence in behavior from module-init-tools. This patch
asserts the following behavior on exit:

* modprobe -r realmod_notloaded => exit zero
* modprobe -r --first-time realmod_notloaded => exit non-zero
* modprobe -r bogusmod => exit non-zero