openvswitch: Fix skb leak using IPv6 defrag

nf_ct_frag6_gather() makes a clone of each skb passed to it, and if the
reassembly is successful, expects the caller to free all of the original
skbs using nf_ct_frag6_consume_orig(). This call was previously missing,
meaning that the original fragments were never freed (with the exception
of the last fragment to arrive).

Fix this by ensuring that all original fragments except for the last
fragment are freed via nf_ct_frag6_consume_orig(). The last fragment
will be morphed into the head, so it must not be freed yet. Furthermore,
retain the ->next pointer for the head after skb_morph().

Fixes: 7f8a436e ("openvswitch: Add conntrack action")
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>