mount: detect the newinstance option for devpts (v2)

The devpts instance was mounted w/o the newinstance option if,
the device number is equal to the root /dev/pts.

I think this condition is strong enough to not mount devpts in a
temporary place.

v2: move the host.bla-bla-bla in kerndat.c
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>