Reduce the target number of vnodes on the freelist from desiredvnodes

Reduce the target number of vnodes on the freelist from desiredvnodes
(usually a couple of thousand) to 25.  The measured impact on cache-hits
doesn't justify spending memory this way:

Target number of free vnodes versus namecache hit rate in % during a
make world:
          10    98.5316
         200    98.5479
         500    98.5546
        1000    98.5709
        3000    98.6006
        4000    98.6126