integrity: support EC-RDSA signatures for asymmetric_verify

Allow to use EC-RDSA signatures for IMA by determining signature type by
the hash algorithm name. This works good for EC-RDSA since Streebog and
EC-RDSA should always be used together.

Cc: Mimi Zohar <zohar@linux.ibm.com>
Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
Cc: linux-integrity@vger.kernel.org
Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>