There was a mountlist race in getnewvnode() whereby the system could block

There was a mountlist race in getnewvnode() whereby the system could block
obtaining the mountlist token while adding a vnode to the mountlist prior
to initializing the vnode's v_usecount and v_data fields.  This bug is
possibly responsible for or related to occassional reports of duplicate
inodes in the system.

Fix the potential problem by more completely initializing the vnode prior
to adding it to the mountlist.  Note that FreeBSD-5 also rearranged thei
r code along the same lines (though this change is independant of their
work).