bridge: start hello_timer when enabling KERNEL_STP in br_stp_start

Since commit 76b91c32dd86 ("bridge: stp: when using userspace stp stop
kernel hello and hold timers"), bridge would not start hello_timer if
stp_enabled is not KERNEL_STP when br_dev_open.

The problem is even if users set stp_enabled with KERNEL_STP later,
the timer will still not be started. It causes that KERNEL_STP can
not really work. Users have to re-ifup the bridge to avoid this.

This patch is to fix it by starting br->hello_timer when enabling
KERNEL_STP in br_stp_start.

As an improvement, it's also to start hello_timer again only when
br->stp_enabled is KERNEL_STP in br_hello_timer_expired, there is
no reason to start the timer again when it's NO_STP.

Fixes: 76b91c32dd86 ("bridge: stp: when using userspace stp stop kernel hello and hold timers")
Reported-by: Haidong Li <haili@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Ivan Vecera <cera@cera.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_stp_if.c

@@ -179,6 +179,7 @@ static void br_stp_start(struct net_bridge *br)
 		br_debug(br, "using kernel STP\n");
 
 		/* To start timers on any ports left in blocking */
+		mod_timer(&br->hello_timer, jiffies + br->hello_time);
 		br_port_state_selection(br);
 	}
 

net/bridge/br_stp_timer.c

@@ -40,7 +40,7 @@ static void br_hello_timer_expired(unsigned long arg)
 	if (br->dev->flags & IFF_UP) {
 		br_config_bpdu_generation(br);
 
-		if (br->stp_enabled != BR_USER_STP)
+		if (br->stp_enabled == BR_KERNEL_STP)
 			mod_timer(&br->hello_timer,
 				  round_jiffies(jiffies + br->hello_time));
 	}