ipv4: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that in some cases I placed the "fall through" comment
on its own line, which is what GCC is expecting to find.

Addresses-Coverity-ID: 115108
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index 6d9b072..e1e2ec0 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -1522,6 +1522,7 @@ static int inetdev_event(struct notifier_block *this, unsigned long event,
 		if (inetdev_valid_mtu(dev->mtu))
 			break;
 		/* disable IP when MTU is not enough */
+		/* fall through */
 	case NETDEV_UNREGISTER:
 		inetdev_destroy(in_dev);
 		break;