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
d8fc6a000f 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

src/nspawn/nspawn.c

@@ -2450,7 +2450,7 @@ static int inner_child(
                 }
         }
 
-        r = mount_all(NULL, arg_userns, true, arg_uid_shift, arg_uid_range, arg_selinux_apifs_context);
+        r = mount_all(NULL, arg_userns, true, arg_uid_shift, arg_private_network, arg_uid_range, arg_selinux_apifs_context);
         if (r < 0)
                 return r;
 
@@ -2705,7 +2705,7 @@ static int outer_child(
                         return log_error_errno(r, "Failed to make tree read-only: %m");
         }
 
-        r = mount_all(directory, arg_userns, false, arg_uid_shift, arg_uid_range, arg_selinux_apifs_context);
+        r = mount_all(directory, arg_userns, false, arg_private_network, arg_uid_shift, arg_uid_range, arg_selinux_apifs_context);
         if (r < 0)
                 return r;