mm: fix cma accounting in zone_watermark_ok

Some cases were reported where atomic unmovable allocations of order 2
fails, but kswapd does not wakeup. And in such cases it was seen that,
when zone_watermark_ok check is  performed to decide whether to wake up
kswapd, there were lot of CMA pages of order 2 and above. This makes
the watermark check succeed resulting in kswapd not being woken up. But
since these atomic unmovable allocations can't come from CMA region,
further atomic allocations keeps failing, without kswapd trying to
reclaim. Usually concurrent movable allocations result in reclaim and
improves the situtation, but the case reported was from a network test
which was resulting in only atomic skb allocations being attempted.

Change-Id: If953b8a8cfb0a5caa1fb63d3c032b194942f8091
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>