librbd: don't decay LUKS{1,2}EncryptionFormat into LUKSEncryptionFormat

librbd: don't decay LUKS{1,2}EncryptionFormat into LUKSEncryptionFormat

Commit 9892ead7fcd9 ("librbd/crypto: allow loading luks format
without specifying version") introduced RBD_ENCRYPTION_FORMAT_LUKS
format identifier, matching cryptsetup's CRYPT_LUKS ("load any LUKS
version happens to be there").  However, in an effort to enable an
obscure "layered encryption with the same passphrase + old QEMU" use
case, it also introduced decaying of RBD_ENCRYPTION_FORMAT_LUKS1 and
RBD_ENCRYPTION_FORMAT_LUKS2 format identifiers, making it impossible
to assert on the format that is being loaded.  This new behavior was
then extended to standalone images.

Treating RBD_ENCRYPTION_FORMAT_LUKS1, RBD_ENCRYPTION_FORMAT_LUKS2
and RBD_ENCRYPTION_FORMAT_LUKS the same when loading encryption can
be construed as an opening for a format downgrade attack.  Let's
resurrect the previous standalone images behavior and extend it to
layered encryption instead.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>