net: decnet: dn_nsp_in: mark expected switch fall-through

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

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/decnet/dn_nsp_in.c

···
 		break;
 	case DN_RUN:
 		sk->sk_shutdown |= SHUTDOWN_MASK;
+		/* fall through */
 	case DN_CC:
 		scp->state = DN_CN;
 	}