KEYS: asymmetric: Fix ECDSA use via keyctl uapi

When support for ECDSA keys was added, constraints for data & signature
sizes were never updated.  This makes it impossible to use such keys via
keyctl API from userspace.

Update constraint on max_data_size to 64 bytes in order to support
SHA512-based signatures. Also update the signature length constraints
per ECDSA signature encoding described in RFC 5480.

Fixes: 299f561a ("x509: Add support for parsing x509 certs with ECDSA keys")
Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>