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>

block/badblocks.c

@@ -525,7 +525,7 @@ ssize_t badblocks_store(struct badblocks *bb, const char *page, size_t len,
 	case 3:
 		if (newline != '\n')
 			return -EINVAL;
-		/* fall through */
+		fallthrough;
 	case 2:
 		if (length <= 0)
 			return -EINVAL;

block/bfq-iosched.c

@@ -4980,7 +4980,7 @@ bfq_set_next_ioprio_data(struct bfq_queue *bfqq, struct bfq_io_cq *bic)
 		pr_err("bdi %s: bfq: bad prio class %d\n",
 				bdi_dev_name(bfqq->bfqd->queue->backing_dev_info),
 				ioprio_class);
-		/* fall through */
+		fallthrough;
 	case IOPRIO_CLASS_NONE:
 		/*
 		 * No prio set, inherit CPU scheduling settings.
@@ -5112,7 +5112,7 @@ static struct bfq_queue **bfq_async_queue_prio(struct bfq_data *bfqd,
 		return &bfqg->async_bfqq[0][ioprio];
 	case IOPRIO_CLASS_NONE:
 		ioprio = IOPRIO_NORM;
-		/* fall through */
+		fallthrough;
 	case IOPRIO_CLASS_BE:
 		return &bfqg->async_bfqq[1][ioprio];
 	case IOPRIO_CLASS_IDLE:

block/blk-wbt.c

@@ -528,7 +528,7 @@ static inline bool wbt_should_throttle(struct rq_wb *rwb, struct bio *bio)
 		if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) ==
 		    (REQ_SYNC | REQ_IDLE))
 			return false;
-		/* fallthrough */
+		fallthrough;
 	case REQ_OP_DISCARD:
 		return true;
 	default:

block/ioprio.c

@@ -71,7 +71,7 @@ int ioprio_check_cap(int ioprio)
 		case IOPRIO_CLASS_RT:
 			if (!capable(CAP_SYS_ADMIN))
 				return -EPERM;
-			/* fall through */
+			fallthrough;
 			/* rt has prio field too */
 		case IOPRIO_CLASS_BE:
 			if (data >= IOPRIO_BE_NR || data < 0)