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/block/badblocks.c b/block/badblocks.c
index 2e5f569..d390566 100644
--- a/block/badblocks.c
+++ b/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;