switch-root: when pivot_root() fails, but old root shall be mounted, do so

switch-root: when pivot_root() fails, but old root shall be mounted, do so

If pivot_root() fails, we'll fall back to switching root via MS_MOVE.
Unlike pivot_root() that won't place the old root fs anyway, but just
hide it. That's problematic during shutdown for example, since after all
we might still want to access it from the exitrd, to disassemble it
properly.

Hence let's make things somewhat systematic: regardless if pivot_root()
or MS_MOVE is used, always make the old root fs show up in the selected
new dir.