BIO - Do not try to redirty a bp on error unless it has a vnode association.

BIO - Do not try to redirty a bp on error unless it has a vnode association.

* vinum buffers may not have a b_vp association.  bdirty() and friends
  requires b_vp to be present.  Simply do not try to re-dirty buffers
  after a failed write unless they have a b_vp association.

  Note that only vinum constructs these manually-kmalloc()'d buffers
  and has this behavior.