fs_mgr: fs_mgr_overlayfs_teardown access to dynamic partition data

If fs_mgr_overlayfs_teardown is called with a specific mount_point
reference (eg: /vendor, /product, /system) when /mnt/scratch is not
mounted (eg: in recovery or fastboot mode), then mount it to perform
the directory teardown.

Test: manual on future changes: fastboot flash vendor -> fastbootd
Bug: 109821105
Bug: 117605276
Change-Id: I5735008e33b57d35345a799dc68bb43c9189135a