Allow autotune maxusers > 384 on 64 bit machines

Allow autotune maxusers > 384 on 64 bit machines

A default install on large memory machines with multiple 10gigE interfaces
were not being given enough mbufs to do full bandwidth TCP or NFS traffic.

To keep the value somewhat reasonable, we scale back the number of
maxuers by 1/6 past the 384 point.  This gives us enough mbufs for most
of our pretty basic 10gigE line-speed tests to complete.