tty: Reset itty for other pty

commit 64e377dcd7d75c241d614458e9619d3445de44ef upstream.

Commit 19ffd68f
('pty: Remove redundant itty reset') introduced a regression
whereby the other pty's linkage is not cleared on teardown.
This triggers a false positive diagnostic in testing.

Properly reset the itty linkage.

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