net/mlx4_en: Refactor mlx4_en_free_tx_desc

net/mlx4_en: Refactor mlx4_en_free_tx_desc

Some code re-ordering, functionally equivalent.

- The !tx_info->inl check is evaluated anyway in both flows
  (common case/end case). Run it first, this might finish
  the flows earlier.
- dma_unmap calls are identical in both flows, get it out
  of the if block into the common area.

Performance tests:
Tested on ConnectX3Pro, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz

Gain is too small to be measurable, no degradation sensed.
Results are similar for IPv4 and IPv6.

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]>