Make EVP_PKEY_bits return 253 for Ed25519.

OpenSSL returns 253, not 256, for both X25519 and Ed25519.
https://crypto.stackexchange.com/questions/62024/x25519-why-openssl-shows-253-bits

Change-Id: Ia1289ed3abaecf79a178476a61af724ac351671d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/36787
Reviewed-by: Adam Langley <[email protected]>

Author: davidben

crypto/evp/p_ed25519_asn1.c

@@ -196,7 +196,7 @@ static int ed25519_priv_encode(CBB *out, const EVP_PKEY *pkey) {
 
 static int ed25519_size(const EVP_PKEY *pkey) { return 64; }
 
-static int ed25519_bits(const EVP_PKEY *pkey) { return 256; }
+static int ed25519_bits(const EVP_PKEY *pkey) { return 253; }
 
 const EVP_PKEY_ASN1_METHOD ed25519_asn1_meth = {
     EVP_PKEY_ED25519,