core/namespace: reimplement mount_private_sysfs() in the same logic to mount private...

core/namespace: reimplement mount_private_sysfs() in the same logic to mount private procfs

Previously, mount_private_sysfs() was implemented by using open_tree()
and move_mount() to keep submounts. But these syscalls are slightly new
and supported since kernel version 5.2.

We already do the same thing for /proc/, but without the new syscalls.
Let's use the same logic to mount private procfs. Then, we can mount
new instance of sysfs with older kernels.