bridge: message age needs to increase, not decrease.

commit bridge: send proper message_age in config BPDU
added this gem:
  bpdu.message_age = (jiffies - root->designated_age)
  p->designated_age = jiffies + bpdu->message_age;
Notice how bpdu->message_age is negated when reassigned to
bpdu.message_age. This causes message age to decrease breaking the
STP protocol.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/bridge/br_stp.c b/net/bridge/br_stp.c
index c9ef3db..6751ed4 100644
--- a/net/bridge/br_stp.c
+++ b/net/bridge/br_stp.c
@@ -186,7 +186,7 @@
 	p->designated_cost = bpdu->root_path_cost;
 	p->designated_bridge = bpdu->bridge_id;
 	p->designated_port = bpdu->port_id;
-	p->designated_age = jiffies + bpdu->message_age;
+	p->designated_age = jiffies - bpdu->message_age;
 
 	mod_timer(&p->message_age_timer, jiffies
 		  + (p->br->max_age - bpdu->message_age));