net_dma: revert 'copied_early'

Now that tcp_dma_try_early_copy() is gone nothing ever sets
copied_early.

Also reverts "53240c20 tcp: Fix possible double-ack w/ user dma"
since it is no longer necessary.

Cc: Ali Saidi <saidi@engin.umich.edu>
Cc: James Morris <jmorris@namei.org>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Neal Cardwell <ncardwell@google.com>
Reported-by: Dave Jones <davej@redhat.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>