powerpc/mm: Fix stupid bug in subpge protection handling

Commit d28513bc ("Fix bug in pagetable
cache cleanup with CONFIG_PPC_SUBPAGE_PROT"), itself a fix for
breakage caused by an earlier clean up patch of mine, contains a
stupid bug.  I changed the parameters of the subpage_protection()
function, but failed to update one of the callers.

This patch fixes it, and replaces a void * with a typed pointer so
that the compiler will warn on such an error in future.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>