[NETFILTER]: nf_nat: fix ICMP translation with statically linked conntrack

When nf_nat/nf_conntrack_ipv4 are linked statically, nf_nat is initialized
before nf_conntrack_ipv4, which makes the nf_ct_l3proto_find_get(AF_INET)
call during nf_nat initialization return the generic l3proto instead of
the AF_INET specific one. This breaks ICMP error translation since the
generic protocol always initializes the IPs in the tuple to 0.

Change the linking order and put nf_conntrack_ipv4 first.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>