sbin/hammer: Fix wrong free big-blocks counter

sbin/hammer: Fix wrong free big-blocks counter

- newfs_hammer needs to decrement extra 2 big-blocks from
  vol0_stat_freebigblocks for zone 8 and 9.

- newfs_hammer has following three lines that allocate the root
  inode, root btree node, and pfs0 metadata for zone 8 and 9,
  however because alloc_blockmap() doesn't decrement vol0_stat
  _freebigblocks when a blockmap first meets a new layer2,
  the volume header shows vol0_stat_freebigblocks 2 more than
  it actually is.

  bnode = alloc_btree_element(&btree_off, &data_buffer0);
  idata = alloc_meta_element(&data_off, sizeof(*idata), &data_buffer1);
  pfsd = alloc_meta_element(&pfsd_off, sizeof(*pfsd), &data_buffer2);

- This also means vol0_stat_bigblocks and vol0_stat_freebigblocks
  are not equal initially. This commit makes vol0_stat_bigblocks
  equal (vol0_stat_freebigblocks + 2), while these two were equal
  until this commit. This should make sense than decrementing
  vol0_stat_bigblocks by 2 to make these two look the same,
  because inodes and btree nodes allocated by regular filesystem
  operations do decrement vol0_stat_freebigblocks whenever a new
  big-block is allocated and those are counted as used big-blocks.
  The initial inode, btree node and pfs shouldn't be an exception.
  The diff of hammer info command right after newfs_hammer would
  look like this.

  ==========
  # diff -aNur ./before ./after
  --- ./before	2015-07-17 03:17:10.999193000 +0900
  +++ ./after	2015-07-17 03:17:14.649193000 +0900
  @@ -1,17 +1,17 @@
  <snip>
   Big-block information
   	Total          178376
  -	Used                0 (0.00%)
  +	Used                2 (0.00%)  /* inode/btree/pfs0 */
   	Reserved           45 (0.03%)
  -	Free           178331 (99.97%)
  +	Free           178329 (99.97%) /* 2 less than before */
   Space information
   	No. Inodes          1
   	Total size       1.4T (1496326340608 bytes)
  -	Used               0B (0.00%)
  +	Used              16M (0.00%)  /* 2x8[MB] more than before */
   	Reserved         360M (0.03%)
   	Free             1.4T (99.97%)
   PFS information
  ==========