mm: compaction: accounting fix

mm: compaction: accounting fix

I saw the following accouting of compaction during test of the series.

compact_blocks_moved 251
compact_pages_moved 44

It's very awkward to me although it's possbile because it means we try to
compact 251 blocks but it just migrated 44 pages.  As further
investigation, I found isolate_migratepages doesn't isolate any pages but
it returns ISOLATE_SUCCESS and then, it just increases
compact_blocks_moved but doesn't increased compact_pages_moved.

This patch makes accouting of compaction works only in case of success of
isolation.

Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <jweiner@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>