lowmemorykiller: also count zram swapfile usage

and tune LMK for use with compressed swap pages

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

Conflicts:
	drivers/staging/android/lowmemorykiller.c

Conflicts:
	drivers/staging/android/lowmemorykiller.c
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>