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>