bonding: use __dev_get_by_name instead of dev_get_by_name to find interface

The following call chain indicates that bond_do_ioctl() is protected
under rtnl_lock. If we use __dev_get_by_name() instead of
dev_get_by_name() to find interface handler in it, this would
help us avoid to change reference counter of interface once.

dev_ioctl()
  rtnl_lock()
  dev_ifsioc()
    bond_do_ioctl()
  rtnl_unlock()

Additionally we also change the coding style in bond_do_ioctl(),
letting it more readable for us.

Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: NYing Xue <ying.xue@windriver.com>
Acked-by: NVeaceslav Falico <vfalico@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -3213,13 +3213,13 @@ static int bond_do_ioctl(struct net_device *bond_dev, struct ifreq *ifr, int cmd
 	if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
 		return -EPERM;
 
-	slave_dev = dev_get_by_name(net, ifr->ifr_slave);
+	slave_dev = __dev_get_by_name(net, ifr->ifr_slave);
 
 	pr_debug("slave_dev=%p:\n", slave_dev);
 
 	if (!slave_dev)
-		res = -ENODEV;
-	else {
+		return -ENODEV;
+
 	pr_debug("slave_dev->name=%s:\n", slave_dev->name);
 	switch (cmd) {
 	case BOND_ENSLAVE_OLD:
@@ -3243,9 +3243,6 @@ static int bond_do_ioctl(struct net_device *bond_dev, struct ifreq *ifr, int cmd
 		res = -EOPNOTSUPP;
 	}
 
-		dev_put(slave_dev);
-	}
-
 	return res;
 }