tweak dirty writeback
different dirty ratios behavior for screen-on / screen-off
smoothness for screen-on, relaxed values for screen-off
mm/page-writeback.c: less dirty memory with screen on
mm/page-writeback.c: adjust dirty writeback for previously introduced lower vmalloc

Ever since vmalloc=182M, all available RAM is low memory. By leaving the
dirty ratios unchanged, this resulted in a massive increase in dirtyable
memory. The new values adjust the ratios to achieve roughly the same amount of
dirtyable memory in megabytes as before.
SELinux: don't check permissions on already freed ressources

Signed-off-by: Andrei F <luxneb@gmail.com>