Fix doubling of rental for PKCS#8 exports in PQC and X25519

src/libraries/Common/src/System/Security/Cryptography/MLDsa.cs

@@ -2150,6 +2150,7 @@ private TResult ExportPkcs8PrivateKeyCallback<TResult>(ExportPkcs8PrivateKeyFunc
 
             while (!TryExportPkcs8PrivateKeyCore(buffer, out written))
             {
+                size = buffer.Length;
                 CryptoPool.Return(buffer);
                 size = checked(size * 2);
                 buffer = CryptoPool.Rent(size);

src/libraries/Common/src/System/Security/Cryptography/MLKem.cs

@@ -1804,6 +1804,7 @@ private TResult ExportPkcs8PrivateKeyCallback<TResult>(ExportPkcs8PrivateKeyFunc
 
             while (!TryExportPkcs8PrivateKeyCore(buffer, out written))
             {
+                size = buffer.Length;
                 CryptoPool.Return(buffer);
                 size = checked(size * 2);
                 buffer = CryptoPool.Rent(size);

src/libraries/Common/src/System/Security/Cryptography/SlhDsa.cs

@@ -1959,6 +1959,7 @@ private TResult ExportPkcs8PrivateKeyCallback<TResult>(ExportPkcs8PrivateKeyFunc
 
             while (!TryExportPkcs8PrivateKeyCore(buffer, out written))
             {
+                size = buffer.Length;
                 CryptoPool.Return(buffer);
                 size = checked(size * 2);
                 buffer = CryptoPool.Rent(size);

src/libraries/System.Security.Cryptography/src/System/Security/Cryptography/X25519DiffieHellman.cs

@@ -1375,6 +1375,7 @@ private TResult ExportPkcs8PrivateKeyCallback<TResult>(Func<ReadOnlySpan<byte>,
 
             while (!TryExportPkcs8PrivateKeyCore(buffer, out written))
             {
+                size = buffer.Length;
                 CryptoPool.Return(buffer);
                 size = checked(size * 2);
                 buffer = CryptoPool.Rent(size);