librbd/crypto: auto detect plaintext parent

librbd/crypto: auto detect plaintext parent

Encryption loading (i.e. rbd_encryption_load) gets a single passphrase
and tries to applies it to all ancestor images. If it fails, the entire load fails.
This commits extends encryption loading to assume ancestor is actually
in plaintext format if no known encryption header magic is detected.

Signed-off-by: Or Ozeri <oro@il.ibm.com>