netfilter: ctnetlink: Resolve conntrack L3-protocol flush regression

Commit 59c08c69 ("netfilter: ctnetlink: Support L3 protocol-filter
on flush") introduced a user-space regression when flushing connection
track entries. Before this commit, the nfgen_family field was not used
by the kernel and all entries were removed. Since this commit,
nfgen_family is used to filter out entries that should not be removed.
One example a broken tool is conntrack. conntrack always sets
nfgen_family to AF_INET, so after 59c08c69 only IPv4 entries were
removed with the -F parameter.

Pablo Neira Ayuso suggested using nfgenmsg->version to resolve the
regression, and this commit implements his suggestion. nfgenmsg->version
is so far set to zero, so it is well-suited to be used as a flag for
selecting old or new flush behavior. If version is 0, nfgen_family is
ignored and all entries are used. If user-space sets the version to one
(or any other value than 0), then the new behavior is used. As version
only can have two valid values, I chose not to add a new
NFNETLINK_VERSION-constant.

Fixes: 59c08c69 ("netfilter: ctnetlink: Support L3 protocol-filter on flush")
Reported-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Tested-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -1254,7 +1254,7 @@ static int ctnetlink_del_conntrack(struct net *net, struct sock *ctnl,
 	struct nf_conntrack_tuple tuple;
 	struct nf_conn *ct;
 	struct nfgenmsg *nfmsg = nlmsg_data(nlh);
-	u_int8_t u3 = nfmsg->nfgen_family;
+	u_int8_t u3 = nfmsg->version ? nfmsg->nfgen_family : AF_UNSPEC;
 	struct nf_conntrack_zone zone;
 	int err;