tty: handle compat PPP ioctls

Multiple tty devices are have tty devices that handle the
PPPIOCGUNIT and PPPIOCGCHAN ioctls. To avoid adding a compat_ioctl
handler to each of those, add it directly in tty_compat_ioctl
so we can remove the calls from fs/compat_ioctl.c.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>