sbin/hammer: Redo e4323571 partly (after reverted by 03d5db37)

sbin/hammer: Redo e4323571 partly (after reverted by 03d5db37)

> sbin/hammer: Fix bug in get_buffer_data()
>
> The previous commit made clear that xor part of get_buffer_data()
> was wrong. Since buf_offset is in any zone not limited to zone-2,
> xor of two offsets doesn't necessarily show the right result to
> know whether they belong to the same buffer, even if ->zone2_offset
> is originally translated from the same zone within the same buffer.
>
> It needs to take xor of long offsets instead of full 64 bits.