powerpc/ppc32: Fixup pmd_page to work when ARCH_PFN_OFFSET is non-zero

Instead of referencing mem_map directly, use pfn_to_page. Otherwise
the kernel crashes when trying to start userspace if ARCH_PFN_OFFSET is
non-zero and CONFIG_BOOKE is not defined

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>