PM / Hibernate / Memory hotplug: Always use for_each_populated_zone()

Use for_each_populated_zone() instead of for_each_zone() in hibernation
code. This fixes a bug on s390, where we allow both config options
HIBERNATION and MEMORY_HOTPLUG, so that we also have a ZONE_MOVABLE
here. We only allow hibernation if no memory hotplug operation was
performed, so in fact both features can only be used exclusively, but
this way we don't need 2 differently configured (distribution) kernels.

If we have an unpopulated ZONE_MOVABLE, we allow hibernation but run
into a BUG_ON() in memory_bm_test/set/clear_bit() because hibernation
code iterates through all zones, not only the populated zones, in
several places. For example, swsusp_free() does for_each_zone() and
then checks for pfn_valid(), which is true even if the zone is not
populated, resulting in a BUG_ON() later because the pfn cannot be
found in the memory bitmap.

Replacing all occurences of for_each_zone() in hibernation code with
for_each_populated_zone() would fix this issue.

[rjw: Rebased on top of linux-next hibernation patches.]

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>