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/udf/symlink.c

@@ -52,7 +52,7 @@ static int udf_pc_to_char(struct super_block *sb, unsigned char *from,
 				elen += pc->lengthComponentIdent;
 				break;
 			}
-			/* Fall through */
+			fallthrough;
 		case 2:
 			if (tolen == 0)
 				return -ENAMETOOLONG;