netfilter: cttimeout: fix slab-out-of-bounds read typo in cttimeout_net_exit

syzbot reports:
  BUG: KASAN: slab-out-of-bounds in __list_del_entry_valid+0xcc/0xf0 lib/list_debug.c:42
  [..]
  list_del include/linux/list.h:148 [inline]
  cttimeout_net_exit+0x211/0x540 net/netfilter/nfnetlink_cttimeout.c:617

Problem is the wrong name of the list member, so container_of() result is wrong.

Reported-by:  <syzbot+92968395eedbdbd3617d@syzkaller.appspotmail.com>
Fixes: 78222bac ("netfilter: cttimeout: decouple unlink and free on netns destruction")
Signed-off-by: Florian Westphal <fw@strlen.de>