powerpc: Get USE_STRICT_MM_TYPECHECKS working again

The typesafe version of the powerpc pagetable handling (with
USE_STRICT_MM_TYPECHECKS defined) has bitrotted again.  This patch
makes a bunch of small fixes to get it back to building status.

It's still not enabled by default as gcc still generates worse
code with it for some reason.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>