openvswitch: Fix double-free on ip_defrag() errors

If ip_defrag() returns an error other than -EINPROGRESS, then the skb is
freed. When handle_fragments() passes this back up to
do_execute_actions(), it will be freed again. Prevent this double free
by never freeing the skb in do_execute_actions() for errors returned by
ovs_ct_execute. Always free it in ovs_ct_execute() error paths instead.

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>