net/mlx5e: CQE based moderation

In this mode the moderation timer will restart upon
new completion (CQE) generation rather than upon interrupt
generation.

The outcome is that for bursty traffic the period timer will never
expire and thus only the moderation frames counter will dictate
interrupt generation, thus the interrupt rate will be relative
to the incoming packets size.
If the burst seizes for "moderation period" time then an interrupt
will be issued immediately.

CQE based moderation is off by default and can be controlled
via ethtool set_priv_flags.

Performance tested on ConnectX4-Lx 50G.

Less packet loss in netperf UDP and TCP tests, with no bw degradation,
for both single and multi streams, with message sizes of
64, 1024, 1472 and 32768 byte.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Gil Rockah <gilr@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>