netfilter: nfnetlink_hook: use the sequence number of the request message

The sequence number allows to correlate the netlink reply message (as
part of the dump) with the original request message.

The cb->seq field is internally used to detect an interference (update)
of the hook list during the netlink dump, do not use it as sequence
number in the netlink dump header.

Fixes: e2cf17d3 ("netfilter: add new hook nfnl subsystem")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>