bgpd: Fix BGP session stuck in OpenConfirm state

Issue:
1. Initially BGP start listening to socket.
2. Start timer expires and BGP tries to connect to peer and moved
   to Idle->connect (lets say peer datastructre X)
3. Connect for X succeeds and hence moved from idle ->connect with
   FD-x.
4. A incoming connection is accepted and a new peer datastructure Y
   is created with FD-y moves from idle->Active state.
5. Peer datastercture Y FD-y sends out OPEN and moves to
   Active->Opensent state.
6. Peer datastrcture Y FD-y receives OPEN and moved from Opensent->
   Openconfirm state.
7. Meanwhile on peer datastrcture X FD-x sends out a OPEN message
   and moved from connect->Opensent.
8. For peer datastrcture Y FD-y keep alive is received and it is
   moved from OpenConfirm->Established.
9. In this case peer datastructure Y FD-y is a accepted connection
   so we try to copy all its parameter to peer datastructure X and
   delete Y.
10. During this process TCP connection for the accepted connection
    (FD-y) goes down and hence get remote address and port fails.
11. With this failure bgp_stop function for both peer datastrure X
    and peer datastructure Y is called.
12. By this time all the parameters include state for datastrcture
    for X and Y are exchanged. Peer Y FD-y when it entered this
    function had state OpenConfirm still which has been moved to peer
    datastrcture X.
13. In bgp_stop it will stop all the timers and take action only if
    peer is in established state. Now that peer datastrcture X and Y
    are not in established state (in this function) it will simply
    close all timers and close the socket and assigns socket for both
    the peer datastrcture to -1.
14. Peer datastrcture Y will be deleted as it is a datastrcture created
    due to accept of connection where as peer datastrcture X will be held
    as it is created with configuration.
15. Now peer datastrcture X now holds a state of OpenConfirm without any
    timers running.
16. With this any new incoming connection will never be able to establish
    as there is config connection X which is stuck in OpenConfirm.

Fix:
 While transferring the peer datastructure Y FD-y (accepted connection)
 to the peer datastructure X, if TCP connection for FD-y goes down, then
 1. Call fsm event bgp_stop for X (do cleanup with bgp_stop and move the
    state to Idle) and
 2. Call fsm event bgp_stop for Y (do cleanup with bgp_stop and gets deleted
    since it is an accept connection).

Signed-off-by: Sarita Patra <saritap@vmware.com>

bgpd/bgp_fsm.c

@@ -304,8 +304,8 @@ static struct peer *peer_xfer_conn(struct peer *from_peer)
 				 ? "accept"
 				 : ""),
 			peer->host, peer->fd, from_peer->fd);
-		bgp_stop(peer);
-		bgp_stop(from_peer);
+		BGP_EVENT_ADD(peer, BGP_Stop);
+		BGP_EVENT_ADD(from_peer, BGP_Stop);
 		return NULL;
 	}
 	if (from_peer->status > Active) {