kpatch: add unload --all command

This may end up being useful in the distro patch module upgrade path.
If the core module also needs updating, we can first do "kpatch unload
--all" and "rmmod kpatch" before doing "kpatch load" of the new patch
module.
Merge pull request #326 from jpoimboe/unload-all

kpatch: add unload --all command