kern_exit race condition problem. this is a hack:
don't vrele() or vgoneall() if we don't have the vnode
any more. no vgoneall() is necessary, because if the refcnt
is zero, there's nobody to take it away from!
a possible "real" long term solution would be to change the extra ref
into a bit in the vnode's flags, and DTRT in the places where the last
ref is killed. That has some problems, too.