caif: __dev_get_by_index instead of dev_get_by_index to find interface

The following call chains indicate that chnl_net_open() is under
rtnl_lock protection as __dev_open() is protected by rtnl_lock.
So if __dev_get_by_index() instead of dev_get_by_index() is used
to find interface handler in it, this would help us avoid to change
interface reference counter.

__dev_open()
  chnl_net_open()

Cc: Dmitry Tarnyagin <dmitry.tarnyagin@lockless.no>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/caif/chnl_net.c

@@ -285,7 +285,7 @@ static int chnl_net_open(struct net_device *dev)
 				goto error;
 		}
 
-		lldev = dev_get_by_index(dev_net(dev), llifindex);
+		lldev = __dev_get_by_index(dev_net(dev), llifindex);
 
 		if (lldev == NULL) {
 			pr_debug("no interface?\n");
@@ -307,7 +307,6 @@ static int chnl_net_open(struct net_device *dev)
 		mtu = min_t(int, dev->mtu, lldev->mtu - (headroom + tailroom));
 		mtu = min_t(int, GPRS_PDP_MTU, mtu);
 		dev_set_mtu(dev, mtu);
-		dev_put(lldev);
 
 		if (mtu < 100) {
 			pr_warn("CAIF Interface MTU too small (%d)\n", mtu);