lowmemorykiller: fix logic derp for ZRAM/ZSWAP enabled use cases

instead of looking at the total available compressed swap, just look at the
free swap pages and take a 50% buffer for it

Signed-off-by: Paul Reioux <reioux@gmail.com>

Conflicts:
	drivers/staging/android/lowmemorykiller.c

	modified:   drivers/staging/android/lowmemorykiller.c

	modified:   drivers/staging/android/lowmemorykiller.c
Change-Id: Ica8f77b4c72de98a0d94f54a51c949415b75f6cf