livepatch: enforce patch stacking semantics

Only allow the topmost patch on the stack to be enabled or disabled, so
that patches can't be removed or added in an arbitrary order.

Suggested-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>