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 <[email protected]>
diff --git a/security/tomoyo/file.c b/security/tomoyo/file.c
index 86f7d1b..051f729 100644
--- a/security/tomoyo/file.c
+++ b/security/tomoyo/file.c
@@ -927,7 +927,7 @@ int tomoyo_path2_perm(const u8 operation, const struct path *path1,
 	case TOMOYO_TYPE_LINK:
 		if (!d_is_dir(path1->dentry))
 			break;
-		/* fall through */
+		fallthrough;
 	case TOMOYO_TYPE_PIVOT_ROOT:
 		tomoyo_add_slash(&buf1);
 		tomoyo_add_slash(&buf2);