memcg: bugfix for memory hotplug

The start pfn calculation in page_cgroup's memory hotplug notifier chain
is wrong.
Tested-by: NBadari Pulavarty <pbadari@us.ibm.com>
Signed-off-by: NKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/page_cgroup.c

@@ -165,7 +165,7 @@ int online_page_cgroup(unsigned long start_pfn,
 	unsigned long start, end, pfn;
 	int fail = 0;
 
-	start = start_pfn & (PAGES_PER_SECTION - 1);
+	start = start_pfn & ~(PAGES_PER_SECTION - 1);
 	end = ALIGN(start_pfn + nr_pages, PAGES_PER_SECTION);
 
 	for (pfn = start; !fail && pfn < end; pfn += PAGES_PER_SECTION) {
@@ -188,7 +188,7 @@ int offline_page_cgroup(unsigned long start_pfn,
 {
 	unsigned long start, end, pfn;
 
-	start = start_pfn & (PAGES_PER_SECTION - 1);
+	start = start_pfn & ~(PAGES_PER_SECTION - 1);
 	end = ALIGN(start_pfn + nr_pages, PAGES_PER_SECTION);
 
 	for (pfn = start; pfn < end; pfn += PAGES_PER_SECTION)