netfilter: nf_conntrack: fix crash when unloading helpers

Commit ea781f19 (netfilter: nf_conntrack: use SLAB_DESTROY_BY_RCU and)
get rid of call_rcu() was missing one conversion to the hlist_nulls
functions, causing a crash when unloading conntrack helper modules.

Reported-and-tested-by: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: Patrick McHardy <kaber@trash.net>