bonding: don't need RTNL for ipsec helpers

The bond_ipsec_* helpers don't need RTNL, and can potentially get called
without it being held, so switch from rtnl_dereference() to
rcu_dereference() to access bond struct data.

Lightly tested with xfrm bonding, no problems found, should address the
syzkaller bug referenced below.

Reported-by: syzbot+582c98032903dcc04816@syzkaller.appspotmail.com
CC: Huy Nguyen <huyn@mellanox.com>
CC: Saeed Mahameed <saeedm@mellanox.com>
CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: "David S. Miller" <davem@davemloft.net>
CC: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
CC: Jakub Kicinski <kuba@kernel.org>
CC: Steffen Klassert <steffen.klassert@secunet.com>
CC: Herbert Xu <herbert@gondor.apana.org.au>
CC: netdev@vger.kernel.org
CC: intel-wired-lan@lists.osuosl.org
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -390,7 +390,7 @@ static int bond_ipsec_add_sa(struct xfrm_state *xs)
 		return -EINVAL;
 
 	bond = netdev_priv(bond_dev);
-	slave = rtnl_dereference(bond->curr_active_slave);
+	slave = rcu_dereference(bond->curr_active_slave);
 	xs->xso.real_dev = slave->dev;
 	bond->xs = xs;
 
@@ -417,7 +417,7 @@ static void bond_ipsec_del_sa(struct xfrm_state *xs)
 		return;
 
 	bond = netdev_priv(bond_dev);
-	slave = rtnl_dereference(bond->curr_active_slave);
+	slave = rcu_dereference(bond->curr_active_slave);
 
 	if (!slave)
 		return;
@@ -442,7 +442,7 @@ static bool bond_ipsec_offload_ok(struct sk_buff *skb, struct xfrm_state *xs)
 {
 	struct net_device *bond_dev = xs->xso.dev;
 	struct bonding *bond = netdev_priv(bond_dev);
-	struct slave *curr_active = rtnl_dereference(bond->curr_active_slave);
+	struct slave *curr_active = rcu_dereference(bond->curr_active_slave);
 	struct net_device *slave_dev = curr_active->dev;
 
 	if (BOND_MODE(bond) != BOND_MODE_ACTIVEBACKUP)