TTY: pty, release tty in all ptmx_open fail paths

Mistakenly, commit 64ba3dc3 (tty: never hold BTM while getting
tty_mutex) switched one fail path in ptmx_open to not free the newly
allocated tty.

Fix that by jumping to the appropriate place. And rename the labels so
that it's clear what is going on there.

Introduced-in: v2.6.36-rc2
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: stable <stable@vger.kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>