Set mark to 0 from libnetfilter_conntrack

Hi,

Damien Boucard from INL has discovered a bug in
libnetfilter_conntrack :
Mark can not be set to 0.

After looking at the code I've found that we only change the
mark if it
is not set to 0 :
	if (ct->mark != 0)
		nfnl_addattr_l(&req->nlh, sizeof(buf), CTA_MARK,
&mark,
		       sizeof(u_int32_t));

What's the cleanest way to solve this. I don't see any mean
to correct
this except adding an IPS_CHANGE_MARK flag.

Proposed patch is attached to the mail.

BR,
-- 
Eric Leblond <ericinl.fr>
INL