n_tty: Reset lnext if canonical mode changes

lnext escapes the next input character as a literal, and must
be reset when canonical mode changes (to avoid misinterpreting
a special character as a literal if canonical mode is changed
back again).

lnext is specifically not reset on a buffer flush so as to avoid
misinterpreting the next input character as a special character.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>