mm: set the watermark_boost before updating the low and high wmarks

Updating the zone watermarks by any means, like extra_free_kbytes,
min_free_kbytes, water_mark_scale_factor, when boost_watermark(zone) is
set can result into the higher water_mark_low|min as the macro
min_wmark_pages returns by adding the zone->watermark_boost.
Set this to Zero early.

Change-Id: I5e3edd1798e46974f3df34ef750f9d08b8ff35f4
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>