net/mlx5: handle Rx CQE compression

net/mlx5: handle Rx CQE compression

Mini (compressed) completion queue entries (CQEs) are returned by the
NIC when PCI back pressure is detected, in which case the first CQE64
contains common packet information followed by a number of CQE8
providing the rest, followed by a matching number of empty CQE64
entries to be used by software for decompression.

Before decompression:

      0           1          2           6         7         8
  +-------+  +---------+ +-------+   +-------+ +-------+ +-------+
  | CQE64 |  |  CQE64  | | CQE64 |   | CQE64 | | CQE64 | | CQE64 |
  |-------|  |---------| |-------|   |-------| |-------| |-------|
  | ..... |  | cqe8[0] | |       | . |       | |       | | ..... |
  | ..... |  | cqe8[1] | |       | . |       | |       | | ..... |
  | ..... |  | ....... | |       | . |       | |       | | ..... |
  | ..... |  | cqe8[7] | |       |   |       | |       | | ..... |
  +-------+  +---------+ +-------+   +-------+ +-------+ +-------+

After decompression:

      0          1     ...     8
  +-------+  +-------+     +-------+
  | CQE64 |  | CQE64 |     | CQE64 |
  |-------|  |-------|     |-------|
  | ..... |  | ..... |  .  | ..... |
  | ..... |  | ..... |  .  | ..... |
  | ..... |  | ..... |  .  | ..... |
  | ..... |  | ..... |     | ..... |
  +-------+  +-------+     +-------+

This patch does not perform the entire decompression step as it would be
really expensive, instead the first CQE64 is consumed and an internal
context is maintained to interpret the following CQE8 entries directly.

Intermediate empty CQE64 entries are handed back to HW without further
processing.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Signed-off-by: Olga Shern <olgas@mellanox.com>
Signed-off-by: Vasily Philipov <vasilyf@mellanox.com>