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/sched/sch_cake.c b/net/sched/sch_cake.c
index 0618b63..7d37638 100644
--- a/net/sched/sch_cake.c
+++ b/net/sched/sch_cake.c
@@ -1670,7 +1670,7 @@ static u32 cake_classify(struct Qdisc *sch, struct cake_tin_data **t,
 		case TC_ACT_QUEUED:
 		case TC_ACT_TRAP:
 			*qerr = NET_XMIT_SUCCESS | __NET_XMIT_STOLEN;
-			/* fall through */
+			fallthrough;
 		case TC_ACT_SHOT:
 			return 0;
 		}