NFSD: Update nfsd_cache_append() to use xdr_stream

When inserting a DRC-cached response into the reply buffer, ensure
that the reply buffer's xdr_stream is updated properly. Otherwise
the server will send a garbage response.

Cc: stable@vger.kernel.org # v6.3+
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>