As reported in nfbz#1820, trying to add a rule and replacing it in the
same batch would crash iptables due to a stale rule pointer left in an
obj_update.
Doing this is perfectly fine in legacy iptables, so implement the
missing feature instead of merely preventing the crash.
Link: https://bugzilla.netfilter.org/show_bug.cgi?id=1820
Fixes: b199aca80d ("nft: Fix leak when replacing a rule")
Signed-off-by: Phil Sutter <phil@nwl.cc>
78d7a5f861