Support ftrace direct callers with livepatch #485

Shung-Hsi Yu reports that BPF fentry progs are no longer invoked if a function is livepatched:
http://lkml.kernel.org/r/rwmwrvvtg3pd7qrnt3of6dideioohwhsplancoc2gdrjran7bg@j5tqng6loymr

Today, we detect livepatch and jump to that before checking if there is a ftrace direct caller. We instead need to prioritize a ftrace direct caller before livepatch. State needs to be setup so that the ftrace direct caller then "returns" to the livepatch-loaded function. For BPF trampolines, this also means that we need to have the trampolines take the function address to call into for F_CALL_ORIG.