mm: ignore the boosting of watermark under lowmemory

When low on free memory and a fragmentation event at the same time
will cause the zone watermarks to be offsetted by watermark_boost
which in default is 150% of high watermark. The side effects of this
is the time taken by the reclaim to reach that level and thus the
subsequent reset of the watermark_boost can be higher and hence
killing of the FG tasks can happen in that time frame despite
the free memory is above zone->_watermark[WMARK_HIGH]. Ignore
boosting of watermark under this condition and thus we can create
more memory headroom.

Change-Id: Ia1fa5d766eee851ad3456f41b4b2d623cf4eeee3
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>