[NETFILTER]: Drop conntrack reference when packet leaves IP

In the event a raw socket is created for sending purposes only, the creator
never bothers to check the socket's receive queue.  But we continue to
add skbs to its queue until it fills up.

Unfortunately, if ip_conntrack is loaded on the box, each skb we add to the
queue potentially holds a reference to a conntrack.  If the user attempts
to unload ip_conntrack, we will spin around forever since the queued skbs
are pinned.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>