init: Add extra_free_kbytes.sh script to adjust watermark_scale_factor

Android uses an out-of-tree /proc/sys/vm/extra_free_kbytes kernel knob
to increase low watermark. A similar /proc/sys/vm/watermark_scale_factor
knob supported in upstream kernel can be used for the same purpose.
The script implements conversion from desired extra_free_kbytes value
to watermark_scale_factor value and sets watermark_scale_factor according
to the value for extra_free_kbytes provided as a parameter.
Note that watermark_scale_factor affects high watermark differently
from extra_free_kbytes. While extra_free_kbytes increases low and high
watermarks by the same amount, watermark_scale_factor affects high
watermark 2x times. Therefore it is expected that replacing the usage
extra_free_kbytes with watermark_scale_factor will keep the same low
watermark but high watermark will be higher than before.

Usage:
    extra_free_kbytes.sh <extra_free_kbytes value>

Bug: 109664768
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Id8949be731da77be884cdeec3a765ea291dbc2f0