RDMA/mlx5: Add modify_op_stat() support

Add support for ib callback modify_op_stat() to add or remove an optional
counter. When adding, a steering flow table is created with a rule that
catches and counts all the matching packets. When removing, the table and
flow counter are destroyed.

Link: https://lore.kernel.org/r/20211008122439.166063-13-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>