mm: forward memory reclaim process

mm: forward memory reclaim process

PD#150206: mm: forward memory reclaim process

memory reclaim process is usually do when first allocate failed
from buddy. This will cause some process crash especially when
memory is low, because there is not enough time to do reclaim
process. This change try to wake up kswapd process to reclaim
memory if free memory is less than high water mark. After apply
this change, free memory is usually keeps high than high_water
mark.

Change-Id: I03b5b931b6ec1fe1345d6f5cf8c15150438a26d3
Signed-off-by: Tao Zeng <tao.zeng@amlogic.com>