When a vnode is vgone()'d its v_ops is replaced with dead_vnode_ops.

When a vnode is vgone()'d its v_ops is replaced with dead_vnode_ops.
dead_vnode_ops replaces VOP_LOCK with a dummy routine that just returns
TRUE.

This blows up anyone actually trying to access the vnode by improperly
returning a successful lock which then panics the machine when the caller
tries to unlock it.  This also screws up VOP_LOCK vs vx_lock() interactions
and can theoretically create other problems.

Normally vgone()'d vnodes have no references and this isn't a problem.
The two notable exceptions are (1) when revoke() is called on a device
(i.e. tty), and (2) when a procfs or linprocfs vnode is destroyed due
to a process exit while another process is accessing it.

Remove dead_lock().  Dead vnodes revert to defaultops which implement
the expected lockmgr lock.