netfilter: nf_tables: use correct lock to protect gc_list

Use nf_tables_gc_list_lock spinlock, not nf_tables_destroy_list_lock to
protect the gc list.

Fixes: 5f68718b

 ("netfilter: nf_tables: GC transaction API to avoid race with control plane")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>