mm: compaction: fix the page state calculation in too_many_isolated

Commit "mm: vmscan: fix the page state calculation in too_many_isolated"
fixed an issue where a number of tasks were blocked in reclaim path
for seconds, because of vmstat_diff not being synced in time.
A similar problem can happen in isolate_migratepages_block, where
similar calculation is performed. This patch fixes that.

Change-Id: Ie74f108ef770da688017b515fe37faea6f384589
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>