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: NNikolay Aleksandrov <nikolay@redhat.com>
Suggested-by: NJay Vosburgh <fubar@us.ibm.com>
Suggested-by: NVeaceslav Falico <vfalico@redhat.com>
Signed-off-by: NDing Tianhong <dingtianhong@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>