cuda.core: a second grow of a grown VMM buffer fails #2908

After one grow a buffer is backed by two physical chunks. The slow path retains only the handle of the mapping at buf.handle and maps aligned_prev_size bytes from it, but cuMemMap requires the size to equal the handle's full size, so the second grow fails. On main the attempt also unmaps the old range first and cannot restore it; with #2880 the old buffer stays intact and the grow raises cleanly. Fix belongs to the ownership redesign (epic #2906).