ppc: Export __handle_mm_fault for MOL

When we did the handle_mm_fault cleanup and get_user_page() race fixes,
handle_mm_fault turned into an inline function that called the real
__handle_mm_fault() code.  The export needed for MOL on ppc wasn't
updated to match the new world order, though.

Turn it into a GPL export while at it, since this is all about internal
interfaces and MOL is GPL'd anwyay.