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>
diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index e56cd56..a475f48 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -610,10 +610,10 @@ static struct sk_buff *veth_xdp_rcv_one(struct veth_rq *rq,
 			goto xdp_xmit;
 		default:
 			bpf_warn_invalid_xdp_action(act);
-			/* fall through */
+			fallthrough;
 		case XDP_ABORTED:
 			trace_xdp_exception(rq->dev, xdp_prog, act);
-			/* fall through */
+			fallthrough;
 		case XDP_DROP:
 			stats->xdp_drops++;
 			goto err_xdp;
@@ -745,10 +745,10 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq,
 		goto xdp_xmit;
 	default:
 		bpf_warn_invalid_xdp_action(act);
-		/* fall through */
+		fallthrough;
 	case XDP_ABORTED:
 		trace_xdp_exception(rq->dev, xdp_prog, act);
-		/* fall through */
+		fallthrough;
 	case XDP_DROP:
 		stats->xdp_drops++;
 		goto xdp_drop;