Limit tty IO chunking to 2kB

The NTTY code can get confused by 4kB chunks, apparently
because n_tty_receive_room() will claim to have more room
than n_tty_receive_buf() can actually accept.

Until somebody figures out what the real n_tty_receive_room()
logic should be, let's just limit it to a safe 2kB.

Thanks go to Andreas Schwab for finding a test-case.

drivers/char/tty_io.c

@@ -1047,8 +1047,13 @@ static inline ssize_t do_tty_write(
 	 *
 	 * But if TTY_NO_WRITE_SPLIT is set, we should use a
 	 * big chunk-size..
+	 *
+	 * The default chunk-size is 2kB, because the NTTY
+	 * layer has problems with bigger chunks. It will
+	 * claim to be able to handle more characters than
+	 * it actually does.
 	 */
-	chunk = 4096;
+	chunk = 2048;
 	if (test_bit(TTY_NO_WRITE_SPLIT, &tty->flags))
 		chunk = 65536;
 	if (count < chunk)