btrfs: zlib: use copy_page for full page copy

The copy_page helper may use an optimized version for full page copy
(eg. on s390 there's a special instruction for that), there's one more
left to convert.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>