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>

arch/powerpc/include/asm/pgtable-ppc32.h

@@ -287,7 +287,7 @@ static inline void __ptep_set_access_flags(pte_t *ptep, pte_t entry)
 #define pmd_page_vaddr(pmd)	\
 	((unsigned long) __va(pmd_val(pmd) & PAGE_MASK))
 #define pmd_page(pmd)		\
-	(mem_map + (pmd_val(pmd) >> PAGE_SHIFT))
+	pfn_to_page(pmd_val(pmd) >> PAGE_SHIFT)
 #else
 #define pmd_page_vaddr(pmd)	\
 	((unsigned long) (pmd_val(pmd) & PAGE_MASK))