rule: expand standalone chain that contains rules

rule: expand standalone chain that contains rules

Otherwise rules that this chain contains are ignored when expressed
using the following syntax:

chain inet filter input2 {
       type filter hook input priority filter; policy accept;
       ip saddr 1.2.3.4 tcp dport { 22, 443, 123 } drop
}

When expanding the chain, remove the rule so the new CMD_OBJ_CHAIN
case does not expand it again.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1655
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>