HAMMER Utility - Change the minimum UNDO/REDO FIFO from 100M to 500M

HAMMER Utility - Change the minimum UNDO/REDO FIFO from 100M to 500M

* The minimum undo/redo fifo really needs to be larger.  Don't play
  around, make it 500M.  People who want to run HAMMER on small hard
  drives or images need to be cognizent of the requirement.

* This partially solves (only partially) a FIFO overflow condition.
  Effectively the complexity of buffered operations hammer allows to
  build up in the kernel could easily overflow a minimally-sized on-media
  UNDO/REDO FIFO.  Upping the requirement makes the case less likely.

  The remainder of the resolution will require some fixes in the
  HAMMER VFS code.

Reported-by: Thomas Nikolajsen <thomas.nikolajsen@mail.dk>