lowmemorykiller: sysfs node and notifications

Create node "/sys/kernel/mm/lowmemkiller/notify_trigger_active",
on which an userspace application can poll for notification when
the number of free physical memory pages in the system falls below
a set threshold. The threshold is set by the new module parameter
"/sys/module/lowmemorykiller/parameters/notify_trigger".
The threshold should be typically set to be higher than the largest
free pages value in /sys/module/lowmemorykiller/parameters/minfree,
so that notification happens before any processes get killed.

Change-Id: Iccb2d1ff699e34b2c22687997e428eec27787b82
Signed-off-by: Manjunatha Madana <mamanj@codeaurora.org>