sbin/hammer: Fix get_buffer_data()'s wrong xor for undo buffer caching

sbin/hammer: Fix get_buffer_data()'s wrong xor for undo buffer caching

As mentioned in 830ab40e
"sbin/hammer: Redo e4323571 partly (after reverted by 03d5db37)",
830ab40e (originally by e4323571) fixed a bug for get_buffer_data()
wrongly taking xor of zone-X offset and zone-2 offset, where X is
one of the direct zones other than zone-2.

This commit fixes it for indirect zone which is zone-3 (and only
zone-3 in this case). The reason zone-3 being different is because
zone-3 to zone-2 translation isn't direct mapped, but instead the
volume header has a pointer to big-blocks that belong to zone-3.

Also see 0942cbbc which made this xor bug clear.