treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

net/tipc/link.c

@@ -1239,7 +1239,7 @@ static bool tipc_data_input(struct tipc_link *l, struct sk_buff *skb,
 			skb_queue_tail(mc_inputq, skb);
 			return true;
 		}
-		/* fall through */
+		fallthrough;
 	case CONN_MANAGER:
 		skb_queue_tail(inputq, skb);
 		return true;