sys/vfs/hammer: Fix wrong big-block accounting

sys/vfs/hammer: Fix wrong big-block accounting

hammer volume-add|del should not count zone4 big-blocks as
a part of vol0_stat_bigblocks. newfs_hammer excludes zone4
big-blocks from vol0_stat_bigblocks on bootstrapping, then
volume-add|del should exclude either.

As the following example shows, it currently makes difference
on Used big-blocks/bytes between
1. newfs_hammer on multiple volumes
and
2. newfs_hammer on a single volume and then add volumes
which isn't something expected to happen.

Example 1.
  # newfs_hammer -L TEST /dev/da1 /dev/da2 /dev/da3 > /dev/null
  # mount_hammer /dev/da1:/dev/da2:/dev/da3 /HAMMER
  # hammer info /HAMMER | grep Used
          Used                2 (0.00%) /* btree + meta */
          Used              16M (0.00%)
  # umount /HAMMER
Example 2.
  # newfs_hammer -L TEST /dev/da1 > /dev/null
  # mount_hammer /dev/da1 /HAMMER
  # hammer volume-add /dev/da2 /HAMMER
  # hammer volume-add /dev/da3 /HAMMER
  # hammer volume-blkdevs /HAMMER
  /dev/da1:/dev/da2:/dev/da3
  # hammer info /HAMMER | grep Used
          Used                4 (0.01%) /* btree + meta + 2freemaps */
          Used              32M (0.01%)
  # umount /HAMMER

With this commit it results the same as the Example 1.
  # newfs_hammer -L TEST /dev/da1 > /dev/null
  # mount_hammer /dev/da1 /HAMMER
  # hammer volume-add /dev/da2 /HAMMER
  # hammer volume-add /dev/da3 /HAMMER
  # hammer volume-blkdevs /HAMMER
  /dev/da1:/dev/da2:/dev/da3
  # hammer info /HAMMER | grep Used
          Used                2 (0.00%)
          Used              16M (0.00%)
  # umount /HAMMER