decnet: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning by explicitly adding a break statement instead of letting the code fall through to the next case.

---
Changes in RESEND:
- None. Resending now that net-next is open.

 net/decnet/dn_route.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 2193ae529e75..37c90c1fdc93 100644
@@ -1407,7 +1407,7 @@ static int dn_route_input_slow(struct sk_buff *skb)
 			flags |= RTCF_DOREDIRECT;
 
 		local_src = DN_FIB_RES_PREFSRC(res);
-
+		break;
 	case RTN_BLACKHOLE:
 	case RTN_UNREACHABLE:
 		break;

2.27.0