rxrpc: Mark expected switch fall-through

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

This patch fixes the following warning:

net/rxrpc/local_object.c: In function ‘rxrpc_open_socket’:
net/rxrpc/local_object.c:175:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (ret < 0) {
      ^
net/rxrpc/local_object.c:184:2: note: here
  case AF_INET:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Currently, GCC is expecting to find the fall-through annotations
at the very bottom of the case and on its own line. That's why
I had to add the annotation, although the intentional fall-through
is already mentioned in a few lines above.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/rxrpc/local_object.c

@@ -180,7 +180,7 @@ static int rxrpc_open_socket(struct rxrpc_local *local, struct net *net)
 		/* Fall through and set IPv4 options too otherwise we don't get
 		 * errors from IPv4 packets sent through the IPv6 socket.
 		 */
-
+		/* Fall through */
 	case AF_INET:
 		/* we want to receive ICMP errors */
 		opt = 1;