sys/vfs/hammer: Make HAMMER_ENCODE() take zone index instead of base

sys/vfs/hammer: Make HAMMER_ENCODE() take zone index instead of base

- HAMMER_ENCODE() macro added in cbf2551 should have taken zone index
  for the first argument, instead of directly importing zone base
  based code from format_blockmap().

- The zone base arg and other two args having overwrapped bits is
  confusing, and not really good for anything. If zone base has non
  zero for non-zone-bits then those bits should go to other two args.

- (format_blockmap() has been changed to take zone/offset instead of
  the encoded one, but no external programs will need this function.
  The same for the HAMMER_ENCODE() macro)