MFC:

MFC:
  Holding a vnode doesn't prevent v_mount from disappearing (when the
  vnode is inactivated), possibly leading to a NULL dereference when
  checking if the mount wants knotes to be activated in the VOP hooks.
  So, we add a new vnode flag VV_NOKNOTE that is only set in  getnewvnode(),
  if necessary, and check it when activating knotes.
  Since the flags are not erased when a vnode is being held, we can safely
  read them.

Approved by:	re (kensmith)