This seems to work as a fix.  With this patch installed, the 
sequence:

cd /some/directory
mount remote-host:/foo /some/directory

fails with EBUSY, as I think it should.

-Keith

*** vfs_syscalls.c.ORIG Wed Oct  5 21:18:28 1994
--- vfs_syscalls.c      Sun Jan 29 21:06:59 1995
***************
*** 120,125 ****
--- 120,131 ----
        }
        if (error = vinvalbuf(vp, V_SAVE, p->p_ucred, p, 0, 0))
                return (error);
+ #ifdef UTK
+       if (vp->v_usecount != 1) {
+               vput (vp);
+               return (EBUSY);
+       }
+ #endif
        if (vp->v_type != VDIR) {
                vput(vp);
                return (ENOTDIR);