x86: stop nmi softlockup warnings in show_mem()

When dumping memory via sysrq-m it is possible to take a bogus NMI
watchdog or softlockup watchdog because the dump can take a long time on
big memory systems.

Occasionally tickle the watchdog when doing the dump.
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/x86/mm/pgtable_32.c

@@ -6,6 +6,7 @@
 #include <linux/kernel.h>
 #include <linux/errno.h>
 #include <linux/mm.h>
+#include <linux/nmi.h>
 #include <linux/swap.h>
 #include <linux/smp.h>
 #include <linux/highmem.h>
@@ -39,6 +40,8 @@ void show_mem(void)
 	for_each_online_pgdat(pgdat) {
 		pgdat_resize_lock(pgdat, &flags);
 		for (i = 0; i < pgdat->node_spanned_pages; ++i) {
+			if (unlikely(i % MAX_ORDER_NR_PAGES == 0))
+				touch_nmi_watchdog();
 			page = pgdat_page_nr(pgdat, i);
 			total++;
 			if (PageHighMem(page))