netfilter: nf_nat: Fix return NF_DROP in nfnetlink_parse_nat_setup

The __nf_nat_alloc_null_binding invokes nf_nat_setup_info which may
return NF_DROP when memory is exhausted, so convert NF_DROP to -ENOMEM
to make ctnetlink happy. Or ctnetlink_setup_nat treats it as a success
when one error NF_DROP happens actully.
Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_nat_core.c

@@ -806,7 +806,7 @@ nfnetlink_parse_nat_setup(struct nf_conn *ct,
 
 	/* No NAT information has been passed, allocate the null-binding */
 	if (attr == NULL)
-		return __nf_nat_alloc_null_binding(ct, manip);
+		return __nf_nat_alloc_null_binding(ct, manip) == NF_DROP ? -ENOMEM : 0;
 
 	err = nfnetlink_parse_nat(attr, ct, &range, l3proto);
 	if (err < 0)