[MIPS] sparsemem: fix crash in show_mem

With sparsemem, pfn should be checked by pfn_valid() before pfn_to_page().
Signed-off-by: NAtsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/mm/pgtable.c

@@ -15,6 +15,8 @@ void show_mem(void)
 	printk("Free swap:       %6ldkB\n", nr_swap_pages<<(PAGE_SHIFT-10));
 	pfn = max_mapnr;
 	while (pfn-- > 0) {
+		if (!pfn_valid(pfn))
+			continue;
 		page = pfn_to_page(pfn);
 		total++;
 		if (PageHighMem(page))