veritysetup: when we fail to unlock a disk with the root hash signature logic, retry...

veritysetup: when we fail to unlock a disk with the root hash signature logic, retry without

Currently, there's no nice way to get a key into the dm-verity kernel
keyring unless recompiling the kernel, or enabling SB or buying into
shim. Neither sounds particularly attractive.

hence provide a reasonable fallback: if unlocking with signed roothash
doesn#t work, just try without. maybe the kernel policy allows this,
maybe not. It's worth a try.