netfilter: nfnetlink_log: always add a timestamp

Compared to all the other work we're already doing to deliver
an skb to userspace this is very cheap - at worse an extra
call to ktime_get_real() - and very useful.

(and indeed it may even be cheaper if we're running from other hooks)

(background: Android occasionally logs packets which
caused wake from sleep/suspend and we'd like to have
timestamps reliably associated with these events)

Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>