devfs - Fix a few more issues (3)

devfs - Fix a few more issues (3)

* devfs_unlinkp() was improperly setting node->v_node to NULL,
  which messes up certain interlocks for race avoidance.

* Use vhold()/vdrop() instead of vref()/vrele() to prevent
  vnode destruction during node teardown.  The node can be
  torndown with an unreferenced vnode.  vref() will panic if
  the refcnt is not already > 0.

* Refactor devfs_vop_reclaim() a bit.  Do not interpose the
  v_release_rdev() call between the node check and the call
  to devfs_freep().