ima: Support EC keys for signature verification

Add support for IMA signature verification for EC keys. Since SHA type
of hashes can be used by RSA and ECDSA signature schemes we need to
look at the key and derive from the key which signature scheme to use.
Since this can be applied to all types of keys, we change the selection
of the encoding type to be driven by the key's signature scheme rather
than by the hash type.

Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: linux-integrity@vger.kernel.org
Cc: David Howells <dhowells@redhat.com>
Cc: keyrings@vger.kernel.org
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Vitaly Chikunov <vt@altlinux.org>
Reviewed-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Acked-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>