bgpd: fd leak in bgpd

bgpd: fd leak in bgpd

* bgp_fsm.c: I have found an fd leak in bgpd that is caused by the 'new'
  Clearing state.  I've been seeing it from hold timer failures, but it can
  also be triggered by other things.

  When Hold_Timer_expired fires in Established state, a notify is sent and
  BGP_Stop event queued.  The fsm then transitions into Clearing state.
  That is the problem; When the BGP_Stop event is serviced, the state table
  says to ignore it while in Clearing.  Thus bgp_stop is not called and the
  fd leaks.  Previously the peer would be in Idle state, which correctly
  handles the BGP_Stop event.

  Fix by making bgp_stop safe to call from Clearing state, without losing
  ClearingCompleted events, and then ensuring it is called prior to
  transition from Clearing->Idle.