team: check return value of team_get_port_by_index_rcu() for NULL

team_get_port_by_index_rcu() might return NULL due to race between port
removal and skb tx path. Panic is easily triggeable when txing packets
and adding/removing port in a loop.

introduced by commit 3d249d4 "net: introduce ethernet teaming device"
and commit 753f993 "team: introduce random mode" (for random mode)

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/team/team_mode_random.c

@@ -28,6 +28,8 @@ static bool rnd_transmit(struct team *team, struct sk_buff *skb)
 
 	port_index = random_N(team->en_port_count);
 	port = team_get_port_by_index_rcu(team, port_index);
+	if (unlikely(!port))
+		goto drop;
 	port = team_get_first_port_txable_rcu(team, port);
 	if (unlikely(!port))
 		goto drop;

drivers/net/team/team_mode_roundrobin.c

@@ -32,6 +32,8 @@ static bool rr_transmit(struct team *team, struct sk_buff *skb)
 
 	port_index = rr_priv(team)->sent_packets++ % team->en_port_count;
 	port = team_get_port_by_index_rcu(team, port_index);
+	if (unlikely(!port))
+		goto drop;
 	port = team_get_first_port_txable_rcu(team, port);
 	if (unlikely(!port))
 		goto drop;