Page allocator: clean up pcp draining functions

- Add comments explaing how drain_pages() works.

- Eliminate useless functions

- Rename drain_all_local_pages to drain_all_pages(). It does drain
  all pages not only those of the local processor.

- Eliminate useless interrupt off / on sequences. drain_pages()
  disables interrupts on its own. The execution thread is
  pinned to processor by the caller. So there is no need to
  disable interrupts.

- Put drain_all_pages() declaration in gfp.h and remove the
  declarations from suspend.h and from mm/memory_hotplug.c

- Make software suspend call drain_all_pages(). The draining
  of processor local pages is may not the right approach if
  software suspend wants to support SMP. If they call drain_all_pages
  then we can make drain_pages() static.

[akpm@linux-foundation.org: fix build]
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: Daniel Walker <dwalker@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/power/snapshot.c

@@ -1203,7 +1203,7 @@ asmlinkage int swsusp_save(void)
 
 	printk(KERN_INFO "PM: Creating hibernation image: \n");
 
-	drain_local_pages();
+	drain_local_pages(NULL);
 	nr_pages = count_data_pages();
 	nr_highmem = count_highmem_pages();
 	printk(KERN_INFO "PM: Need to copy %u pages\n", nr_pages + nr_highmem);
@@ -1221,7 +1221,7 @@ asmlinkage int swsusp_save(void)
 	/* During allocating of suspend pagedir, new cold pages may appear.
 	 * Kill them.
 	 */
-	drain_local_pages();
+	drain_local_pages(NULL);
 	copy_data_pages(&copy_bm, &orig_bm);
 
 	/*