bonding: create bond_first_slave_rcu()

The bond_first_slave_rcu() will be used to instead of bond_first_slave()
in rcu_read_lock().

According to the Jay Vosburgh's suggestion, the struct netdev_adjacent
should hide from users who wanted to use it directly. so I package a
new function to get the first slave of the bond.

Suggested-by: Nikolay Aleksandrov <nikolay@redhat.com>
Suggested-by: Jay Vosburgh <fubar@us.ibm.com>
Suggested-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/bonding/bonding.h b/drivers/net/bonding/bonding.h
index 8283cbd..8f0d6d0 100644
--- a/drivers/net/bonding/bonding.h
+++ b/drivers/net/bonding/bonding.h
@@ -101,6 +101,10 @@
 		netdev_adjacent_get_private(bond_slave_list(bond)->prev) : \
 		NULL)
 
+/* Caller must have rcu_read_lock */
+#define bond_first_slave_rcu(bond) \
+	netdev_lower_get_first_private_rcu(bond->dev)
+
 #define bond_is_first_slave(bond, pos) (pos == bond_first_slave(bond))
 #define bond_is_last_slave(bond, pos) (pos == bond_last_slave(bond))