xfrm: skip policies marked as dead while rehashing

syzkaller triggered following KASAN splat:

BUG: KASAN: slab-out-of-bounds in xfrm_hash_rebuild+0xdbe/0xf00 net/xfrm/xfrm_policy.c:618
read of size 2 at addr ffff8801c8e92fe4 by task kworker/1:1/23 [..]
Workqueue: events xfrm_hash_rebuild [..]
 __asan_report_load2_noabort+0x14/0x20 mm/kasan/report.c:428
 xfrm_hash_rebuild+0xdbe/0xf00 net/xfrm/xfrm_policy.c:618
 process_one_work+0xbbf/0x1b10 kernel/workqueue.c:2112
 worker_thread+0x223/0x1990 kernel/workqueue.c:2246 [..]

The reproducer triggers:
1016                 if (error) {
1017                         list_move_tail(&walk->walk.all, &x->all);
1018                         goto out;
1019                 }

in xfrm_policy_walk() via pfkey (it sets tiny rcv space, dump
callback returns -ENOBUFS).

In this case, *walk is located the pfkey socket struct, so this socket
becomes visible in the global policy list.

It looks like this is intentional -- phony walker has walk.dead set to 1
and all other places skip such "policies".

Ccing original authors of the two commits that seem to expose this
issue (first patch missed ->dead check, second patch adds pfkey
sockets to policies dumper list).

Fixes: 880a6fab ("xfrm: configure policy hash table thresholds by netlink")
Fixes: 12a169e7 ("ipsec: Put dumpers on the dump list")
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Timo Teras <timo.teras@iki.fi>
Cc: Christophe Gouault <christophe.gouault@6wind.com>
Reported-by: syzbot <bot+c028095236fcb6f4348811565b75084c754dc729@syzkaller.appspotmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>