netfilter: ctnetlink: fix double helper assignation for NAT'ed conntracks

If we create a conntrack that has NAT handlings and a helper, the helper
is assigned twice. This happens because nf_nat_setup_info() - via
nf_conntrack_alter_reply() - sets the helper before ctnetlink, which
indeed does not check if the conntrack already has a helper as it thinks that
it is a brand new conntrack.

The fix moves the helper assignation before the set of the status flags.
This avoids a bogus assertion in __nf_ct_ext_add (if netfilter assertions are
enabled) which checks that the conntrack must not be confirmed.

This problem was introduced in 2.6.23 with the netfilter extension
infrastructure.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>