net: some optimizations in af_inet

1) Use eq_net() in inet_netns_ok() to speedup socket creation if
   !CONFIG_NET_NS

2) Reorder the tests about inet_ehash_secret generation (once only)
   Use the unlikely() macro when testing if inet_ehash_secret already
   generated.

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