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/core/pktgen.c

@@ -3430,7 +3430,7 @@ xmit_more:
 		net_info_ratelimited("%s xmit error: %d\n",
 				     pkt_dev->odevname, ret);
 		pkt_dev->errors++;
-		/* fall through */
+		fallthrough;
 	case NETDEV_TX_BUSY:
 		/* Retry it next time */
 		refcount_dec(&(pkt_dev->skb->users));