openvswitch: Fix helper reference leak

If the actions (re)allocation fails, or the actions list is larger than the
maximum size, and the conntrack action is the last action when these
problems are hit, then references to helper modules may be leaked. Fix
the issue.

Fixes: cae3a262 ("openvswitch: Allow attaching helpers to ct action")
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>