Work around vnode reuse bug resulting in a panic: vop_generic_badop

Work around vnode reuse bug resulting in a panic: vop_generic_badop

Joel hit this frequently on the go builder, and this was
also found by szykiller

https://syzkaller.appspot.com/bug?extid=58bdde9f7a1a407514a7
https://syzkaller.appspot.com/bug?extid=5779bc64fc4fdd0a5140

This is based on a workaround originally done by visa@ and mbuhl@
but not committed or widely distributed.

Realistically this should be fixed more like the previous attempt
with vdoom, but my attempts to do this at the moment are colliding
with finding all sources of similar races, now that kernel unlocking
is exposing these previously existing bugs

for now, let's put in this ugly workaround

ok deraadt@