netfilter: conntrack: dccp, sctp: handle null timeout argument

The timeout pointer can be NULL which means we should modify the
per-nets timeout instead.

All do this, except sctp and dccp which instead give:

general protection fault: 0000 [#1] PREEMPT SMP KASAN
net/netfilter/nf_conntrack_proto_dccp.c:682
 ctnl_timeout_parse_policy+0x150/0x1d0 net/netfilter/nfnetlink_cttimeout.c:67
 cttimeout_default_set+0x150/0x1c0 net/netfilter/nfnetlink_cttimeout.c:368
 nfnetlink_rcv_msg+0xcf2/0xfb0 net/netfilter/nfnetlink.c:229
 netlink_rcv_skb+0x177/0x450 net/netlink/af_netlink.c:2477

Reported-by:  <syzbot+46a4ad33f345d1dd346e@syzkaller.appspotmail.com>
Fixes: c779e849

 ("netfilter: conntrack: remove get_timeout() indirection")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>