btrfs: use for loop in prealloc_file_extent_cluster

This function iterates all extents in the extent cluster, make this
intention obvious by using a for loop. No functional chanes.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>