net/mlx5: qos: Maintain rate group vport members in a list

Previously, finding group members was done by iterating over all vports
of an eswitch and comparing their group with the required one, but that
approach will break down when a group can contain vports from multiple
eswitches.

Solve that by maintaining a list of vport members.
Instead of iterating over esw vports, loop over the members list.
Use this opportunity to provide two new functions to allocate and free a
group, so that the number of state transitions is smaller. This will
also be used in a future patch.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>