homework: Always upload volume key to keyring

homework: Always upload volume key to keyring

This commit makes homework always upload the LUKS volume key into the
kernel keyring. This is different from previous behavior in three
notable ways:

- Previously, we'd only upload if auto-resize was on. In preparation for
upcoming changes, now we always upload

- Previously, we'd upload the user's actual password (or a password
obtained from a FIDO key or similar). Now, we upload the LUKS volume key
itself, to remove a layer of unnecessary indirection.

- Previously, Lock() wouldn't remove the key from the kernel keyring.
This, of course, defeats the purpose of Lock(), so now it removes the
key

This commit also allows the LUKS volume to be unlocked using the volume
key we obtained from the keyring.