core/namespace: mount new sysfs when new network namespace is requested

core/namespace: mount new sysfs when new network namespace is requested

Even when a mount namespace is created, previously host's sysfs is used,
especially with RootDirectory= or RootImage=, thus service processes can
still access the properties of the network interfaces in the main network
namespace through sysfs.

This makes, sysfs is remounted with the new network namespace tag, except
when PrivateMounts= is explicitly disabled. Hence, the properties of the
network interfaces in the main network namespace cannot be accessed by
service processes through sysfs.

Fixes #26422.