bridge: fix bridge netlink RCU usage

When the STP timer fires, it can call br_ifinfo_notify(),
which in turn ends up in the new br_get_link_af_size().
This function is annotated to be using RTNL locking, which
clearly isn't the case here, and thus lockdep warns:

  ===============================
  [ INFO: suspicious RCU usage. ]
  3.19.0+ #569 Not tainted
  -------------------------------
  net/bridge/br_private.h:204 suspicious rcu_dereference_protected() usage!

Fix this by doing RCU locking here.

Fixes: b7853d73e39b ("bridge: add vlan info to bridge setlink and dellink notification messages")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>