sparsemem: fix oops in x86_64 show_mem

We aren't sampling for holes in memory. Thus we encounter a section hole with
empty section map pointer for SPARSEMEM and OOPs for show_mem. This issue
has been seen in 2.6.21, current git and current mm. This patch is for
2.6.21 stable. It was tested against sparsemem.

Previous to commit f0a5a58a memory_present
was called for node_start_pfn to node_end_pfn. This would cover the hole(s)
with reserved pages and valid sections. Most SPARSEMEM supported arches
do a pfn_valid check in show_mem before computing the page structure address.

This issue was brought to my attention on IRC by Arnaldo Carvalho de Melo at
acme@redhat.com. Thanks to Arnaldo for testing.
Signed-off-by: Bob Picco <bob.picco@hp.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

arch/x86_64/mm/init.c

@@ -72,6 +72,8 @@ void show_mem(void)
 
 	for_each_online_pgdat(pgdat) {
                for (i = 0; i < pgdat->node_spanned_pages; ++i) {
+			if (!pfn_valid(pgdat->node_start_pfn + i))
+				continue;
 			page = pfn_to_page(pgdat->node_start_pfn + i);
 			total++;
 			if (PageReserved(page))