livepatch: Introduce klp_for_each_patch macro

There are already macros to iterate over struct klp_func and klp_object.

Add also klp_for_each_patch(). But make it internal because also
klp_patches list is internal.

Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>