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To support multi-user emulated storage, we mount rootfs as MS_SHARED,
which means we can't MS_MOVE existing mount points rooted in the
shared subtree.  Initial staging is still able to MS_MOVE, since it's
rooted in a MS_PRIVATE tmpfs rooted at /mnt/secure.

This change fixes unmounting by operating in-place instead of trying
(and failing) to MS_MOVE back to staging.

Bug: 7127564
Change-Id: I4783db4319b61c0915da39361cbc7e8f4943d094