bonding: convert primary_slave to use RCU

This is necessary mainly for two bonding call sites: procfs and
sysfs as it was dereferenced without any real protection.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/bonding/bond_netlink.c b/drivers/net/bonding/bond_netlink.c
index e1489d9..c13d83e 100644
--- a/drivers/net/bonding/bond_netlink.c
+++ b/drivers/net/bonding/bond_netlink.c
@@ -443,6 +443,7 @@
 	unsigned int packets_per_slave;
 	int ifindex, i, targets_added;
 	struct nlattr *targets;
+	struct slave *primary;
 
 	if (nla_put_u8(skb, IFLA_BOND_MODE, BOND_MODE(bond)))
 		goto nla_put_failure;
@@ -492,9 +493,9 @@
 			bond->params.arp_all_targets))
 		goto nla_put_failure;
 
-	if (bond->primary_slave &&
-	    nla_put_u32(skb, IFLA_BOND_PRIMARY,
-			bond->primary_slave->dev->ifindex))
+	primary = rtnl_dereference(bond->primary_slave);
+	if (primary &&
+	    nla_put_u32(skb, IFLA_BOND_PRIMARY, primary->dev->ifindex))
 		goto nla_put_failure;
 
 	if (nla_put_u8(skb, IFLA_BOND_PRIMARY_RESELECT,