block/block-copy: fix s->copy_size for compressed cluster

0e240245 allowed writes larger than cluster, but that's
unsupported for compressed write. Fix it.

Fixes: 0e240245
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20191029150934.26416-1-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>