ipv4: Use math to point per net sysctls into the appropriate struct net.

Simplify maintenance of ipv4_net_table by using math to point the per
net sysctls into the appropriate struct net, instead of manually
reassinging all of the variables into hard coded table slots.

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