Comment 0 for bug 1808097

[Impact]
The agetty stopped working using serial tty when user logged out, especially if there is a remained process. The agetty failed to open tty device due to serial port used by previous process was not released.

[Test Case]
1. Run background processes from serial console
$ while true; do sleep 10; done &

2. Log out
$ logout

3. Read journal log by journalctl | tail
Jan 28 16:07:01 ubuntu systemd[1]: Stopped Serial Getty on ttyAMA0.
Jan 28 16:07:01 ubuntu systemd[1]: Started Serial Getty on ttyAMA0.
Jan 28 16:07:02 ubuntu agetty[1643]: /dev/ttyAMA0: not a tty

[Fix]
The upstream has a fix since 4.20-rc6, which turns off TTY_IO_ERROR flag if port comes from console. The commit link is provided by

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/tty/tty_port.c?h=v4.20-rc6&id=2a48602615e0a2f563549c7d5c8d507f904cf96e

[Regression Potential]
Should be none, and it's regression becoz 4.4 kernel didn't have this case.