SUNRPC: Clean up svcauth_gss_release()

Now that upper layers use an xdr_stream to track the construction
of each RPC Reply message, resbuf->len is kept up-to-date
automatically. There's no need to recompute it in svc_gss_release().

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>