Revert "shutdown: do not umount recursively before MS_MOVE"

Revert "shutdown: do not umount recursively before MS_MOVE"

This reverts commit 6b219b74de53729249956221a971047aab7c96e0.

This commit doesn't look right to me. We have to unmount everything
recursively *before* we MS_MOVE because the MS_MOVE will not get rid of
it for us, and we simply cannot access these mounts after the MS_MOVE is
complete anymore.

This is a fundamental difference between MS_MOVE and pivot_root(). The
latter repivots the entire mount table getting rid of anything outside
of the new root. MS_MOVE otoh just mounts a bunch of mount points to the
top, leaving in place whatever might be underneath it.

Thus, if we go through the MS_MOVE codepath we must unmount everything
explicitly before doing so because otherwise the mounts will be pinned
forever, but be entirely invisble to userspace.