netfilter: synproxy_core: fix warning in __nf_ct_ext_add_length()

With CONFIG_NETFILTER_DEBUG we get the following warning during SYNPROXY init:

[   80.558906] WARNING: CPU: 1 PID: 4833 at net/netfilter/nf_conntrack_extend.c:80 __nf_ct_ext_add_length+0x217/0x220 [nf_conntrack]()

The reason is that the conntrack template is set to confirmed before adding
the extension and it is invalid to add extensions to already confirmed
conntracks. Fix by adding the extensions before setting the conntrack to
confirmed.

Reported-by: Jesper Dangaard Brouer <jesper.brouer@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>