netfilter: fix crash during boot if NAT has been compiled built-in

(c7232c99 netfilter: add protocol independent NAT core) introduced a
problem that leads to crashing during boot due to NULL pointer
dereference. It seems that xt_nat calls xt_register_target() before
xt_init():

net/netfilter/x_tables.c:static struct xt_af *xt; is NULL and we crash on
xt_register_target(struct xt_target *target)
{
        u_int8_t af = target->family;
        int ret;

        ret = mutex_lock_interruptible(&xt[af].mutex);
...

Fix this by changing the linking order, to make sure that x_tables
comes before xt_nat.
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/Makefile

@@ -47,7 +47,6 @@ nf_nat-y	:= nf_nat_core.o nf_nat_proto_unknown.o nf_nat_proto_common.o \
 		   nf_nat_proto_udp.o nf_nat_proto_tcp.o nf_nat_helper.o
 
 obj-$(CONFIG_NF_NAT) += nf_nat.o
-obj-$(CONFIG_NF_NAT) += xt_nat.o
 
 # NAT protocols (nf_nat)
 obj-$(CONFIG_NF_NAT_PROTO_DCCP) += nf_nat_proto_dccp.o
@@ -71,6 +70,7 @@ obj-$(CONFIG_NETFILTER_XTABLES) += x_tables.o xt_tcpudp.o
 obj-$(CONFIG_NETFILTER_XT_MARK) += xt_mark.o
 obj-$(CONFIG_NETFILTER_XT_CONNMARK) += xt_connmark.o
 obj-$(CONFIG_NETFILTER_XT_SET) += xt_set.o
+obj-$(CONFIG_NF_NAT) += xt_nat.o
 
 # targets
 obj-$(CONFIG_NETFILTER_XT_TARGET_AUDIT) += xt_AUDIT.o