PM / Hibernate: Make default image size depend on total RAM size

The default hibernation image size is currently hard coded and euqal
to 500 MB, which is not a reasonable default on many contemporary
systems.  Make it equal 2/5 of the total RAM size (this is slightly
below the maximum, i.e. 1/2 of the total RAM size, and seems to be
generally suitable).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: M. Vefa Bicakci <bicave@superonline.com>
diff --git a/kernel/power/main.c b/kernel/power/main.c
index 0a28d4d..f06ad6e 100644
--- a/kernel/power/main.c
+++ b/kernel/power/main.c
@@ -321,6 +321,7 @@
 	int error = pm_start_workqueue();
 	if (error)
 		return error;
+	hibernate_image_size_init();
 	power_kobj = kobject_create_and_add("power", NULL);
 	if (!power_kobj)
 		return -ENOMEM;