mm/compaction.c:isolate_freepages_block(): small tuneup

mm/compaction.c:isolate_freepages_block(): small tuneup

- remove unneeded `continue'

- expand the scope if the `if (isloated)' test, to optimise a code path
  which is rarely actually taken.

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Laura Abbott <lauraa@codeaurora.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>