tipc: clear 'next'-pointer of message fragments before reassembly

If the 'next' pointer of the last fragment buffer in a message is not
zeroed before reassembly, we risk ending up with a corrupt message,
since the reassembly function itself isn't doing this.

Currently, when a buffer is retrieved from the deferred queue of the
broadcast link, the next pointer is not cleared, with the result as
described above.

This commit corrects this, and thereby fixes a bug that may occur when
long broadcast messages are transmitted across dual interfaces. The bug
has been present since 40ba3cdf ("tipc:
message reassembly using fragment chain")

This commit should be applied to both net and net-next.
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/bcast.c

@@ -559,6 +559,7 @@ void tipc_bclink_rcv(struct sk_buff *buf)
 
 		buf = node->bclink.deferred_head;
 		node->bclink.deferred_head = buf->next;
+		buf->next = NULL;
 		node->bclink.deferred_size--;
 		goto receive;
 	}