netfilter: nf_tables: release flow rule object from commit path

No need to postpone this to the commit release path, since no packets
are walking over this object, this is accessed from control plane only.
This helped uncovered UAF triggered by races with the netlink notifier.

Fixes: 9dd732e0 ("netfilter: nf_tables: memleak flow rule from commit path")
Reported-by:  <syzbot+8f747f62763bc6c32916@syzkaller.appspotmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>