tcp_metrics: panic when tcp_metrics_init fails.

There is not a practical way to cleanup during boot so
just panic if there is a problem initializing tcp_metrics.

That will at least give us a clear place to start debugging
if something does go wrong.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>