sbin/hammer: Remove unnecessary blockmap argument

sbin/hammer: Remove unnecessary blockmap argument

- format_freemap() does not need the second argument for blockmap.
  This function sets that blockmap pointer to
  &root_vol->ondisk->vol0_blockmap[4];
  right before it starts to initialize blockmap anyway.

- Also passing the freemap blockmap as an argument is redundant
  in terms of api since format_freemap() can access the freemap
  blockmap via the first argument (root volume, as mentioned above)
  given that the function knows it's looking for blockmaps[4].