net/mlx4_en: Improve XDP xmit function

net/mlx4_en: Improve XDP xmit function

Several performance improvements in XDP TX datapath,
including:
- Ring a single doorbell for XDP TX ring per NAPI budget,
  instead of doing it per a lower threshold (was 8).
  This includes removing the flow of immediate doorbell ringing
  in case of a full TX ring.
- Compiler branch predictor hints.
- Calculate values in compile time rather than in runtime.

Performance tests:
Tested on ConnectX3Pro, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
Single queue no-RSS optimization ON.

XDP_TX packet rate:
-------------------------------------
     | Before    | After     | Gain |
IPv4 | 10.3 Mpps | 12.0 Mpps |  17% |
IPv6 | 10.3 Mpps | 12.0 Mpps |  17% |
-------------------------------------

Signed-off-by: Tariq Toukan <[email protected]>
Reviewed-by: Saeed Mahameed <[email protected]>
Cc: [email protected]
Cc: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>