net: net_secret should not depend on TCP

A host might need net_secret[] and never open a single socket.

Problem added in commit aebda156a570782
("net: defer net_secret[] initialization")

Based on prior patch from Hannes Frederic Sowa.

Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@strressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>