tty: pty: Fix ldisc flush after userspace become aware of the data already

While using emacs, cat or others' commands in konsole with recent
kernels, I have met many times that CTRL-C freeze konsole. After
konsole freeze I can't type anything, then I have to open a new one,
it is very annoying.

See bug report:
https://bugs.kde.org/show_bug.cgi?id=175283



The platform in that bug report is Solaris, but now the pty in linux
has the same problem or the same behavior as Solaris :)

It has high possibility to trigger the problem follow steps below:
Note: In my test, BigFile is a text file whose size is bigger than 1G
1:open konsole
1:cat BigFile
2:CTRL-C

After some digging, I find out the reason is that commit 1d1d14da
("pty: Fix buffer flush deadlock") changes the behavior of pty_flush_buffer.

Thread A                                 Thread B
--------                                 --------
1:n_tty_poll return POLLIN
                                         2:CTRL-C trigger pty_flush_buffer
                                             tty_buffer_flush
                                               n_tty_flush_buffer
3:attempt to check count of chars:
  ioctl(fd, TIOCINQ, &available)
  available is equal to 0

4:read(fd, buffer, avaiable)
  return 0

5:konsole close fd

Yes, I know we could use the same patch included in the BUG report as
a workaround for linux platform too. But I think the data in ldisc is
belong to application of another side, we shouldn't clear it when we
want to flush write buffer of this side in pty_flush_buffer. So I think
it is better to disable ldisc flush in pty_flush_buffer, because its new
hehavior bring no benefit except that it mess up the behavior between
POLLIN, and TIOCINQ or FIONREAD.

Also I find no flush_buffer function in others' tty driver has the
same behavior as current pty_flush_buffer.

Fixes: 1d1d14da ("pty: Fix buffer flush deadlock")
CC: stable@vger.kernel.org # v4.0+
Signed-off-by: Wang YanQing <udknight@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>