common/mlx5: query preserve capability via DevX

common/mlx5: query preserve capability via DevX

Update function mlx5_devx_cmd_query_hca_attr() to add the
reg_c_preserve bit query.

The stored metadata in register C may be lost in NIC Tx and
FDB egress while doing one of the following operations:
 - packet encapsulation.
 - packet mirroring (multiple processing paths).
 - packet sampling (using Flow Sampler).

If the reg_c_preserve bit is set to 1, then the above
limitation is obsolete, the all metadata registers Cx
preserve their values even through the operations mentioned
above.

Signed-off-by: Jiawei Wang <jiaweiw@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>