net: netrom: nr_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/netrom/nr_in.c

···
 
 	case NR_DISCREQ:
 		nr_write_internal(sk, NR_DISCACK);
-
+		/* fall through */
 	case NR_DISCACK:
 		nr_disconnect(sk, 0);
 		break;