kernel - reduces kern.maxvnodes on machines with less memory

kernel - reduces kern.maxvnodes on machines with less memory

* Increase the divisor a bit to compensate for the fact that HAMMER
  eats significantly more kernel memory than UFS on a per-vnode basis.
  This is to prevent the VM system from allocating all available memory
  for required kernel structures on machines with small amounts of memory,
  such as 128M, on vkernels, and on virtual machines configured with
  fairly low amounts of memory.