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>

kernel/debug/kdb/kdb_keyboard.c

@@ -173,11 +173,11 @@ int kdb_get_kbd_char(void)
 	case KT_LATIN:
 		if (isprint(keychar))
 			break;		/* printable characters */
-		/* fall through */
+		fallthrough;
 	case KT_SPEC:
 		if (keychar == K_ENTER)
 			break;
-		/* fall through */
+		fallthrough;
 	default:
 		return -1;	/* ignore unprintables */
 	}