net/mlx4: mitigate Tx send entry size calculations

net/mlx4: mitigate Tx send entry size calculations

The previuse code took a send queue entry size for stamping from the
send queue entry pointed by completion queue entry; This 2 reads were
done per packet in completion stage.

The completion burst packets number is managed by fixed size stored in
Tx queue, so we can infer that each valid completion entry actually frees
the next fixed number packets.

The descriptors ring holds the send queue entry, so we just can infer
all the completion burst packet entries size by simple calculation and
prevent calculations per packet.

Adjust completion functions to free full completion bursts packets
by one time and prevent per packet work queue entry reads and
calculations.

Save only start of completion burst or Tx burst send queue entry
pointers in the appropriate descriptor element.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>