netfilter: conntrack: initialize ct->timeout

KMSAN started reporting an error when accessing ct->timeout for the
first time without initialization:

 BUG: KMSAN: uninit-value in __nf_ct_refresh_acct+0x1ae/0x470 net/netfilter/nf_conntrack_core.c:1765
 ...
 dump_stack+0x173/0x1d0 lib/dump_stack.c:113
 kmsan_report+0x131/0x2a0 mm/kmsan/kmsan.c:624
 __msan_warning+0x7a/0xf0 mm/kmsan/kmsan_instr.c:310
 __nf_ct_refresh_acct+0x1ae/0x470 net/netfilter/nf_conntrack_core.c:1765
 nf_ct_refresh_acct ./include/net/netfilter/nf_conntrack.h:201
 nf_conntrack_udp_packet+0xb44/0x1040 net/netfilter/nf_conntrack_proto_udp.c:122
 nf_conntrack_handle_packet net/netfilter/nf_conntrack_core.c:1605
 nf_conntrack_in+0x1250/0x26c9 net/netfilter/nf_conntrack_core.c:1696
 ...
 Uninit was created at:
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:205
 kmsan_internal_poison_shadow+0x92/0x150 mm/kmsan/kmsan.c:159
 kmsan_kmalloc+0xa9/0x130 mm/kmsan/kmsan_hooks.c:173
 kmem_cache_alloc+0x554/0xb10 mm/slub.c:2789
 __nf_conntrack_alloc+0x16f/0x690 net/netfilter/nf_conntrack_core.c:1342
 init_conntrack+0x6cb/0x2490 net/netfilter/nf_conntrack_core.c:1421
Signed-off-by: Alexander Potapenko <glider@google.com>
Fixes: cc169213 ("netfilter: conntrack: avoid same-timeout update")
Cc: Florian Westphal <fw@strlen.de>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_core.c

@@ -1350,6 +1350,7 @@ __nf_conntrack_alloc(struct net *net,
 	/* save hash for reusing when confirming */
 	*(unsigned long *)(&ct->tuplehash[IP_CT_DIR_REPLY].hnnode.pprev) = hash;
 	ct->status = 0;
+	ct->timeout = 0;
 	write_pnet(&ct->ct_net, net);
 	memset(&ct->__nfct_init_offset[0], 0,
 	       offsetof(struct nf_conn, proto) -