specfs: Remove specnode from hash table in spec_node_revoke.

Previously, it was possible for spec_node_lookup_by_dev to handle a
speconde that a concurrent spec_node_destroy is about to remove from
the hash table and then free, as soon as spec_node_lookup_by_dev
releases device_lock.

Now, the ordering is:

1. Remove specnode from hash table in spec_node_revoke.  At this
  point, no _new_ vnode references are possible (other than possibly
  one acquired by vcache_vget under v_interlock), but there may be
  existing ones.

2. Mark vnode reclaimed so vcache_vget will fail.

3. The last vrele (or equivalent logic in vcache_vget) will then free
  the specnode in spec_node_destroy.

This way, _if_ a thread in spec_node_lookup_by_dev finds a specnode
in the hash table under device_lock/v_interlock, _then_ it will not
be freed until the thread completes vcache_vget.

This change requires calling spec_node_revoke unconditionally for
device special nodes, not just for active ones.  Might introduce
slightly more contention on device_lock but not much because we
already have to take it in this path anyway a little later in
spec_node_destroy.

(riastradh)