RDMA/mlx5: Support optional counters in hw_stats initialization

Add optional counter support when allocate and initialize hw_stats
structure. Optional counters have IB_STAT_FLAG_OPTIONAL flag set and are
disabled by default.

Link: https://lore.kernel.org/r/20211008122439.166063-11-markzhang@nvidia.com


Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>