powerpc: Avoid integer overflow in page_is_ram()

Commit 8b15047 ("ppc: make phys_mem_access_prot() work with pfns
instead of addresses") fixed page_is_ram() in arch/ppc to avoid overflow
for addresses above 4G on 32-bit kernels.  However arch/powerpc's
page_is_ram() is missing the same fix -- it computes a physical address
by doing pfn << PAGE_SHIFT, which overflows if pfn corresponds to a page
above 4G.

In particular this causes pages above 4G to be mapped with the wrong
caching attribute; for example many ppc440-based SoCs have PCI space
above 4G, and mmap()ing MMIO space may end up with a mapping that has
caching enabled.

Fix this by working with the pfn and avoiding the conversion to
physical address that causes the overflow.  This patch compares the
pfn to max_pfn, which is a semantic change from the old code -- that
code compared the physical address to high_memory, which corresponds
to max_low_pfn.  However, I think that was is another bug, since
highmem pages are still RAM.

Reported-by: vb <vb@vsbe.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/mem.c

@@ -75,11 +75,10 @@ static inline pte_t *virt_to_kpte(unsigned long vaddr)
 
 int page_is_ram(unsigned long pfn)
 {
-	unsigned long paddr = (pfn << PAGE_SHIFT);
-
 #ifndef CONFIG_PPC64	/* XXX for now */
-	return paddr < __pa(high_memory);
+	return pfn < max_pfn;
 #else
+	unsigned long paddr = (pfn << PAGE_SHIFT);
 	int i;
 	for (i=0; i < lmb.memory.cnt; i++) {
 		unsigned long base;