net: bridge: vlan: account for router port lists when notifying

When sending a global vlan notification we should account for the number
of router ports when allocating the skb, otherwise we might end up
losing notifications.

Fixes: dc002875

 ("net: bridge: vlan: use br_rports_fill_info() to export mcast router ports")
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>