lowmemorykiller: Account for ZONE_MOVEABLE

The lowmemorykiller should kill userspace processes if the available free
memory for gfp_mask is below minfree.  For GFP_KERNEL, memory in
ZONE_MOVEABLE is not useable and should be discounted.

Since we are now considering a subset of total memory, adjust minfree
proportionally before comparing.

Change-Id: Ibdcf0a42f7f52688282766ea632002a715318c8b
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>