netfilter: cttimeout: fix deadlock due to erroneous unlock/lock conversion

The spin_unlock call should have been left as-is, revert.

Fixes: b16c2919 ("netfilter: nf_conntrack: use safer way to lock all buckets")
Reported-by: kernel test robot <fengguang.wu@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>