kernel - Fix rare vref() assertion

kernel - Fix rare vref() assertion

* The VREF_TERMINATE flag gets cleared when a vnode is reactivated.
  However, concurrent LK_SHARED locks on vnodes can race the v_state
  test.  Thus the code cannot assume that VREF_TERMINATE has been cleared
  when v_state is VS_ACTIVE.

  To avoid the race, we simply unconditionally clear VREF_TERMINATE on
  a successful vget().

* Could be reproduced by running blogbench and synth together, both of
  which generate extreme filesystem-intensive loads.