net/mlx5: DR, Add support for dumping steering info

Extend mlx5 debugfs support to present Software Steering resources:
dr_domain including it's tables, matchers and rules.
The interface is read-only. While dump is being presented, new steering
rules cannot be inserted/deleted.

The steering information is dumped in the CSV form with the following
format:

    <object_type>,<object_ID>, <object_info>,...,<object_info>

This data can be read at the following path:

    /sys/kernel/debug/mlx5/<BDF>/steering/fdb/<domain_handle>

Example:

    # cat /sys/kernel/debug/mlx5/0000:82:00.0/steering/fdb/dmn_000018644
    3100,0x55caa4621c50,0xee802,4,65533
    3101,0x55caa4621c50,0xe0100008

Changes in V2:
 - Reduce temp hex buffer size and avoid unnecessary memset
 - Use bin2hex() instead of DIY loop
 - Don't check debugfs functions return values

Signed-off-by: Muhammad Sammar <muhammads@nvidia.com>
Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>