nspawn: skip /sys-as-tmpfs if we don't use private-network

nspawn: skip /sys-as-tmpfs if we don't use private-network

Since v3.11/7dc5dbc ("sysfs: Restrict mounting sysfs"), the kernel
doesn't allow mounting sysfs if you don't have CAP_SYS_ADMIN rights over
the network namespace.

So the mounting /sys as a tmpfs code introduced in
d8fc6a000fe21b0c1ba27fbfed8b42d00b349a4b doesn't work with user
namespaces if we don't use private-net. The reason is that we mount
sysfs inside the container and we're in the network namespace of the host
but we don't have CAP_SYS_ADMIN over that namespace.

To fix that, we mount /sys as a sysfs (instead of tmpfs) if we don't use
private network and ignore the /sys-as-a-tmpfs code if we find that /sys
is already mounted as sysfs.

Fixes #1555