hammer2 - Refactor reserved block selection in freemap code

hammer2 - Refactor reserved block selection in freemap code

* Refactor the reserved block selection in the freemap code.  Move from
  4 copies of each freemap block to 15 copies in order to ensure
  that any of the four volume header backups (which are rotated on each
  flush) can be used at mount-time.

* A better algorithm could use as few as 10 copies but for now I am using
  a more trivial algorithm which needs 15.

* No media changes, the 4MB/2GB of space already reserved had sufficient
  room.