mm/compaction: check migrate_pages's return value instead of list_empty()

Many migrate_page's caller check return value instead of list_empy by
cf608ac1

 ("mm: compaction: fix COMPACTPAGEFAILED counting").  This patch
makes compaction's migrate_pages consistent with others.  This patch
should not change old behavior.

Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>