kernel - Close MP race in vnode allocation code

kernel - Close MP race in vnode allocation code

* vx_lock_nonblock() is used by allocfreevnode() to interlock the
  vnode being freed.  However, this function will incorrect succeed
  on a vnode recursively held by a caller of allocfreevnode() which
  is in the middle of being reclaimed if the vnode in question
  allows LK_CANRECURSE locks in the lockinit.  UFS vnodes use this
  mechanic.

  Add a little bit of code to close the hole.