kmod/core: fix kpatch_num_registered decrement

Oops, kpatch_unregister doesn't decrement kpatch_num_registered whenever
it's > 1.
kmod/core: kpatch_unregister return error if !kpmod->enabled

It's probably cleaner to just return -EINVAL if the kpmod isn't enabled,
instead of warning and continuing, which would be dangerous.

I think the reason I had it WARN before is because this condition
shouldn't be possible given the source for the patch module.  But the
core module can't necessarily assume that it's our trustworthy patch
module code on the other side.
Merge pull request #169 from jpoimboe/unregister-fix

kmod/core: fixes in the unregister path