n_tty_flush_buffer can happen in parallel with n_tty_close that the
tty->disc_data will be set to NULL. n_tty_flush_buffer accesses
tty->disc_data, so we must prevent n_tty_close clear tty->disc_data
while n_tty_flush_buffer has a non-NULL view of tty->disc_data.
So we need to make sure that accesses to disc_data are atomic using
tty->termios_rwsem.
There is an example I meet:
When n_tty_flush_buffer accesses tty struct, the disc_data is right.
However, then reset_buffer_flags accesses tty->disc_data, disc_data
become NULL, So kernel crash when accesses tty->disc_data->real_tail.
I guess there could be another thread change tty->disc_data to NULL,
and during N_TTY line discipline, n_tty_close will set tty->disc_data
to be NULL. So use tty->termios_rwsem to protect disc_data between close
and flush_buffer.