mount: use correct mount namespace in open_mountpoint

open_mountpoint() have to operate in target's mount namespace,
which may differ from root's namespace - root_item->pid.real.

So obtain the correct mount namespace from mount_info and use
it in switch_ns().

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>