There's no need to fchdir() out of the rootfs and back into it around the umount2(), hence don't. This brings the logic closer to what the pivot_root() man page suggests. While we are at it, always operate based on fds, once we opened the original dir, and pass the path string along only for generating messages (i.e. as "decoration"). Add tests for both code paths: the pivot_root() one and the MS_MOUNT.
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