[NET]: inet_ehash_secret should be __read_mostly and set only once

There is a very tiny probability that build_ehash_secret() is called
at the same time by different CPUS.

Also, using __read_mostly is a must for inet_ehash_secret

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>