nspawn: mount temporary visible procfs and sysfs instance

nspawn: mount temporary visible procfs and sysfs instance

In order to mount procfs and sysfs in an unprivileged container the
kernel requires that a fully visible instance is already present in the
target mount namespace. Mount one here so the inner child can mount its
own  instances. Later we umount the temporary  instances created here
before we actually exec the payload. Since the rootfs is shared the
umount will propagate into the container. Note, the inner child wouldn't
be able to unmount the  instances on its own since it doesn't own the
originating mount namespace. IOW, the outer child needs to do this.

So far nspawn didn't run into this issue because it used MS_MOVE which
meant that the shadow mount tree pinned a procfs and sysfs instance
which the kernel would find. The shadow mount tree is gone with proper
pivot_root() semantics.

Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>