net/mlx5: introduce buffer size parameter for hairpin

net/mlx5: introduce buffer size parameter for hairpin

When creating a hairpin queue, the total data size and the maximal
number of packets are interrelated. The differ is the stride size.
Larger buffer size means big packet like jumbo could be supported,
but in the meanwhile, it will introduce more cache misses and have a
side effect on the performance.
Now a new device parameter "hp_buf_log_sz" is introduced for
applications to set the total data buffer size (the logarithm value).
Then the maximal number of packets will also be calculated
automatically by this value.
Applications could also change this value to a larger one in order
to support larger packets in hairpin case. A smaller value will be
beneficial for memory consumption.
If it is not set, the default value will be used.

Signed-off-by: Bing Zhao <bingz@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>