Use the blk_device supplied by vdc encryptFstab

fs_mgr may put other dm devices on top of the raw disk, such as for
checkpointing, and it hands metadata encryption the uppermost device in
vdc. That's what should be encrypted, not the raw disk.

Bug: 150354860
Test: Treehugger
Change-Id: I279f087b1b7aded40c5a62281154851ce970ba70