tty: wipe buffer if not echoing data

If we are not echoing the data to userspace or the console is in icanon
mode, then perhaps it is a "secret" so we should wipe it once we are
done with it.

This mirrors the logic that the audit code has.

Reported-by: aszlig <aszlig@nix.build>
Tested-by: Milan Broz <gmazyland@gmail.com>
Tested-by: Daniel Zatovic <daniel.zatovic@gmail.com>
Tested-by: aszlig <aszlig@nix.build>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>