net/mlx4: mitigate Tx path memory barriers

net/mlx4: mitigate Tx path memory barriers

Replace most of the memory barriers by IO memory barriers since they
are all targeted to the DRAM; This improves code efficiency for
systems which force store order between different addresses.

Only the doorbell register store should be protected by memory barrier
since it is targeted to the PCI memory domain.

Limit pre byte count store IO memory barrier for systems with cache
line size smaller than 64B (TXBB size).

This patch improves Tx performance by 0.2MPPS for one segment 64B
packets via 1 queue with 1 core test.

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