netfilter: nfnetlink_queue: cleanup copy_range usage

For every packet queued, we check if configured copy_range
is 0, and treat that as 'copy entire packet'.

We can move this check to the queue configuration, and can
set copy_range appropriately.

Also, convert repetitive '0xffff - NLA_HDRLEN' to a macro.

[ queue initialization still used 0xffff, although its harmless
  since the initial setting is overwritten on queue config ]

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>