FORMAT CHANGE - include start address explicitly in indirect blocks.

FORMAT CHANGE - include start address explicitly in indirect blocks.

By storing the address of the first block in an indirect block,
that block can be relocated in the index tree with no adverse effects.
This makes it easier to handle blocks that have become empty.

So an indirect block now contains:
   u16 0x01   'indirect' indicator
   u32 addr   virtual address of first indexed block
   u48 paddr  physical address of first indexblock block
   u48 paddr2 next block ... etc