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 f40d1e42bb98 ("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This patch removes this outdated statement.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 <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>