bonding: fix suspicious RCU usage in bond_ipsec_add_sa()

To dereference bond->curr_active_slave, it uses rcu_dereference().
But it and the caller doesn't acquire RCU so a warning occurs.
So add rcu_read_lock().

Test commands:
    ip link add dummy0 type dummy
    ip link add bond0 type bond
    ip link set dummy0 master bond0
    ip link set dummy0 up
    ip link set bond0 up
    ip x s add proto esp dst 14.1.1.1 src 15.1.1.1 spi 0x07 \
	    mode transport \
	    reqid 0x07 replay-window 32 aead 'rfc4106(gcm(aes))' \
	    0x44434241343332312423222114131211f4f3f2f1 128 sel \
	    src 14.0.0.52/24 dst 14.0.0.70/24 proto tcp offload \
	    dev bond0 dir in

Splat looks like:
=============================
WARNING: suspicious RCU usage
5.13.0-rc3+ #1168 Not tainted
-----------------------------
drivers/net/bonding/bond_main.c:411 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by ip/684:
 #0: ffffffff9a2757c0 (&net->xfrm.xfrm_cfg_mutex){+.+.}-{3:3},
at: xfrm_netlink_rcv+0x59/0x80 [xfrm_user]
   55.191733][  T684] stack backtrace:
CPU: 0 PID: 684 Comm: ip Not tainted 5.13.0-rc3+ #1168
Call Trace:
 dump_stack+0xa4/0xe5
 bond_ipsec_add_sa+0x18c/0x1f0 [bonding]
 xfrm_dev_state_add+0x2a9/0x770
 ? memcpy+0x38/0x60
 xfrm_add_sa+0x2278/0x3b10 [xfrm_user]
 ? xfrm_get_policy+0xaa0/0xaa0 [xfrm_user]
 ? register_lock_class+0x1750/0x1750
 xfrm_user_rcv_msg+0x331/0x660 [xfrm_user]
 ? rcu_read_lock_sched_held+0x91/0xc0
 ? xfrm_user_state_lookup.constprop.39+0x320/0x320 [xfrm_user]
 ? find_held_lock+0x3a/0x1c0
 ? mutex_lock_io_nested+0x1210/0x1210
 ? sched_clock_cpu+0x18/0x170
 netlink_rcv_skb+0x121/0x350
 ? xfrm_user_state_lookup.constprop.39+0x320/0x320 [xfrm_user]
 ? netlink_ack+0x9d0/0x9d0
 ? netlink_deliver_tap+0x17c/0xa50
 xfrm_netlink_rcv+0x68/0x80 [xfrm_user]
 netlink_unicast+0x41c/0x610
 ? netlink_attachskb+0x710/0x710
 netlink_sendmsg+0x6b9/0xb70
[ ... ]

Fixes: 18cb261afd ("bonding: support hardware encryption offload to slaves")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -403,10 +403,12 @@ static int bond_ipsec_add_sa(struct xfrm_state *xs)
 	struct net_device *bond_dev = xs->xso.dev;
 	struct bonding *bond;
 	struct slave *slave;
+	int err;
 
 	if (!bond_dev)
 		return -EINVAL;
 
+	rcu_read_lock();
 	bond = netdev_priv(bond_dev);
 	slave = rcu_dereference(bond->curr_active_slave);
 	xs->xso.real_dev = slave->dev;
@@ -415,10 +417,13 @@ static int bond_ipsec_add_sa(struct xfrm_state *xs)
 	if (!(slave->dev->xfrmdev_ops
 	      && slave->dev->xfrmdev_ops->xdo_dev_state_add)) {
 		slave_warn(bond_dev, slave->dev, "Slave does not support ipsec offload\n");
+		rcu_read_unlock();
 		return -EINVAL;
 	}
 
-	return slave->dev->xfrmdev_ops->xdo_dev_state_add(xs);
+	err = slave->dev->xfrmdev_ops->xdo_dev_state_add(xs);
+	rcu_read_unlock();
+	return err;
 }
 
 /**