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/net/core/dev.c b/net/core/dev.c
index d42c9ea..b9c6f31 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4690,10 +4690,10 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 		break;
 	default:
 		bpf_warn_invalid_xdp_action(act);
-		/* fall through */
+		fallthrough;
 	case XDP_ABORTED:
 		trace_xdp_exception(skb->dev, xdp_prog, act);
-		/* fall through */
+		fallthrough;
 	case XDP_DROP:
 	do_drop:
 		kfree_skb(skb);