tcp: reflect SYN queue_mapping into SYNACK packets

While testing how linux behaves on SYNFLOOD attack on multiqueue device
(ixgbe), I found that SYNACK messages were dropped at Qdisc level
because we send them all on a single queue.

Obvious choice is to reflect incoming SYN packet @queue_mapping to
SYNACK packet.

Under stress, my machine could only send 25.000 SYNACK per second (for
200.000 incoming SYN per second). NIC : ixgbe with 16 rx/tx queues.

After patch, not a single SYNACK is dropped.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Hans Schillstrom <hans.schillstrom@ericsson.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>