Staging: android: lowmemorykiller: Only iterate over process list when needed.

Use NR_ACTIVE plus NR_INACTIVE as a size estimate for our fake cache
instead the sum of rss. Neither method is accurate.

Also skip the process scan, if the amount of memory available is above
the largest threshold set.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Cc: David Rientjes <rientjes@google.com>
Cc: San Mehat <san@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>