netfilter: ctnetlink: don't use conntrack/expect object addresses as id

commit 3c791076 upstream.

else, we leak the addresses to userspace via ctnetlink events
and dumps.

Compute an ID on demand based on the immutable parts of nf_conn struct.

Another advantage compared to using an address is that there is no
immediate re-use of the same ID in case the conntrack entry is freed and
reallocated again immediately.

Fixes: 35832402 ("[NETFILTER]: nf_conntrack_expect: kill unique ID")
Fixes: 7f85f914

 ("[NETFILTER]: nf_conntrack: kill unique ID")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
[bwh: Backported to 3.16:
 - Include <net/netns/hash.h> in nf_conntrack_core.c
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>