BugLink: https://bugs.launchpad.net/bugs/1813873

Try to get reference for ldisc during tty_reopen().
If ldisc present, we don't need to do tty_ldisc_reinit() and lock the
write side for line discipline semaphore.
Effectively, it optimizes fast-path for tty_reopen(), but more
importantly it won't interrupt ongoing IO on the tty as no ldisc change
is needed.
Fixes user-visible issue when tty_reopen() interrupted login process for
user with a long password, observed and reported by Lukas.

Fixes: c96cf923 ("tty: Don't block on IO when ldisc change is pending")
Fixes: 83d817f4 ("tty: Hold tty_ldisc_lock() during tty_reopen()")
Cc: Jiri Slaby <jslaby@suse.com>
Reported-by: Lukas F. Hartmann <lukas@mntmn.com>
Tested-by: Lukas F. Hartmann <lukas@mntmn.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(cherry picked from commit d3736d82)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>