netfilter: nfnetlink_queue: assert monotonic packet ids

Packet identifier is currently setup in nfqnl_build_packet_message(),
using one atomic_inc_return().

Problem is that since several cpus might concurrently call
nfqnl_enqueue_packet() for the same queue, we can deliver packets to
consumer in non monotonic way (packet N+1 being delivered after packet
N)

This patch moves the packet id setup from nfqnl_build_packet_message()
to nfqnl_enqueue_packet() to guarantee correct delivery order.

This also removes one atomic operation.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Florian Westphal <fw@strlen.de>
CC: Pablo Neira Ayuso <pablo@netfilter.org>
CC: Eric Leblond <eric@regit.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>