netfilter: nfnetlink_log: validate dependencies to avoid breaking atomicity

Check that dependencies are fulfilled before updating the logger
instance, otherwise we can leave things in intermediate state on errors
in nfulnl_recv_config().

[ Ken-ichirou reports that this is also fixing missing instance refcnt drop
  on error introduced in his patch 914eebf2 ("netfilter: nfnetlink_log:
  autoload nf_conntrack_netlink module NFQA_CFG_F_CONNTRACK config flag"). ]

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Tested-by: Ken-ichirou MATSUZAWA <chamaken@gmail.com>