kpatch: change core module path again

On RHEL I'm seeing issues with putting the core module in the "extra"
path.  On the next depmod run, it gets added to modules.dep, and on a
subsequent kpatch install I see the following errors:

    /usr/lib/dracut/modules.d/50drm/module-setup.sh: line 26: /lib/modules/3.10.0-123.4.4.el7.x86_64//weak-updates/kpatch/kpatch.ko: No such file or directory
    /usr/lib/dracut/modules.d/90kernel-modules/module-setup.sh: line 14: /lib/modules/3.10.0-123.4.4.el7.x86_64//weak-updates/kpatch/kpatch.ko: No such file or directory
    modinfo: ERROR: Module /lib/modules/3.10.0-123.4.4.el7.x86_64/weak-updates/kpatch/kpatch.ko not found.

Until the core module gets merged into Linux, I think we can put it in
/usr/lib/kpatch, which is also where the patch modules are going to be
delivered in the RHEL RPM.

Making sure the other options still work with the kpatch utility for
now, so as to keep backwards compatibility between a newer kpatch
utility and older core modules.  We can break this compatibility for
kpatch 0.2.0.
Merge pull request #337 from jpoimboe/core-module-path

kpatch: change core module path again