mm/compaction.c: update comment about zone lock in isolate_freepages_block

mm/compaction.c: update comment about zone lock in isolate_freepages_block

Since commit f40d1e4 ("mm: compaction: acquire the zone->lock as late as
possible"), isolate_freepages_block() takes the zone->lock itself.  The
function description however still states that the zone->lock must be
held.

This patch removes this outdated statement.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>