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.

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/decnet/dn_route.c

@@ -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;