net sysctl: Initialize the network sysctls sooner to avoid problems.

If the netfilter code is modified to use register_net_sysctl_table the
kernel fails to boot because the per net sysctl infrasturce is not setup
soon enough.  So to avoid races call net_sysctl_init from sock_init().

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Acked-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>