tipc: fix issues with early FAILOVER_MSG from peer

It appears that a FAILOVER_MSG can come from peer even when the failure
link is resetting (i.e. just after the 'node_write_unlock()'...). This
means the failover procedure on the node has not been started yet.
The situation is as follows:

         node1                                node2
  linkb          linka                  linka        linkb
    |              |                      |            |
    |              |                      x failure    |
    |              |                  RESETTING        |
    |              |                      |            |
    |              x failure            RESET          |
    |          RESETTING             FAILINGOVER       |
    |              |   (FAILOVER_MSG)     |            |
    |<-------------------------------------------------|
    | *FAILINGOVER |                      |            |
    |              | (dummy FAILOVER_MSG) |            |
    |------------------------------------------------->|
    |            RESET                    |            | FAILOVER_END
    |         FAILINGOVER               RESET          |
    .              .                      .            .
    .              .                      .            .
    .              .                      .            .

Once this happens, the link failover procedure will be triggered
wrongly on the receiving node since the node isn't in FAILINGOVER state
but then another link failover will be carried out.
The consequences are:

1) A peer might get stuck in FAILINGOVER state because the 'sync_point'
was set, reset and set incorrectly, the criteria to end the failover
would not be met, it could keep waiting for a message that has already
received.

2) The early FAILOVER_MSG(s) could be queued in the link failover
deferdq but would be purged or not pulled out because the 'drop_point'
was not set correctly.

3) The early FAILOVER_MSG(s) could be dropped too.

4) The dummy FAILOVER_MSG could make the peer leaving FAILINGOVER state
shortly, but later on it would be restarted.

The same situation can also happen when the link is in PEER_RESET state
and a FAILOVER_MSG arrives.

The commit resolves the issues by forcing the link down immediately, so
the failover procedure will be started normally (which is the same as
when receiving a FAILOVER_MSG and the link is in up state).

Also, the function "tipc_node_link_failover()" is toughen to avoid such
a situation from happening.

Acked-by: Jon Maloy <jon.maloy@ericsson.se>
Signed-off-by: Tuong Lien <tuong.t.lien@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>