crypto/keys/internal: use crypto/rand.Reader for generating private key (#8742)

genPrivKey rejects invalid fieldelems, so we must use a real reader
instead of the zero reader.

Fixes #8741

crypto/keys/internal/benchmarking/bench.go

@@ -1,6 +1,7 @@
 package benchmarking
 
 import (
+	"crypto/rand"
 	"io"
 	"testing"
 
@@ -13,22 +14,12 @@ import (
 // Use of this source code is governed by a BSD-style
 // license that can be found at the bottom of this file.
 
-type zeroReader struct{}
-
-func (zeroReader) Read(buf []byte) (int, error) {
-	for i := range buf {
-		buf[i] = 0
-	}
-	return len(buf), nil
-}
-
 // BenchmarkKeyGeneration benchmarks the given key generation algorithm using
 // a dummy reader.
 func BenchmarkKeyGeneration(b *testing.B, generateKey func(reader io.Reader) types.PrivKey) {
 	b.ReportAllocs()
-	var zero zeroReader
 	for i := 0; i < b.N; i++ {
-		generateKey(zero)
+		generateKey(rand.Reader)
 	}
 }