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>

drivers/net/tun.c

@@ -1590,10 +1590,10 @@ static int tun_xdp_act(struct tun_struct *tun, struct bpf_prog *xdp_prog,
 		break;
 	default:
 		bpf_warn_invalid_xdp_action(act);
-		/* fall through */
+		fallthrough;
 	case XDP_ABORTED:
 		trace_xdp_exception(tun->dev, xdp_prog, act);
-		/* fall through */
+		fallthrough;
 	case XDP_DROP:
 		this_cpu_inc(tun->pcpu_stats->rx_dropped);
 		break;
@@ -2417,7 +2417,7 @@ static int tun_xdp_one(struct tun_struct *tun,
 		switch (err) {
 		case XDP_REDIRECT:
 			*flush = true;
-			/* fall through */
+			fallthrough;
 		case XDP_TX:
 			return 0;
 		case XDP_PASS: