common/mlx5: introduce user index field in completion

common/mlx5: introduce user index field in completion

On ConnectX devices the completion entry provides the dedicated 24-bit
field, that is filled up with some static value assigned at the
Receiving Queue creation moment. This patch declares this field. This is
a preparation step for supporting shared RQs and the field is supposed
to provide actual port index while handling the shared receiving
queue(s).

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>