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>

fs/ext2/super.c

@@ -587,7 +587,7 @@ static int parse_options(char *options, struct super_block *sb,
 		case Opt_xip:
 			ext2_msg(sb, KERN_INFO, "use dax instead of xip");
 			set_opt(opts->s_mount_opt, XIP);
-			/* Fall through */
+			fallthrough;
 		case Opt_dax:
 #ifdef CONFIG_FS_DAX
 			ext2_msg(sb, KERN_WARNING,