net/mlx5: qos: Store rate groups in a qos domain

Groups are currently maintained as a list in their corresponding
eswitch, protected by the esw state_lock.
The upcoming cross-eswitch scheduling feature cannot work with this
approach, as it would require acquiring multiple eswitch locks (in the
correct order) in order to maintain group membership.

This commit moves the rate groups into a new 'qos domain' struct and
adds explicit qos init/cleanup steps to the eswitch init/cleanup.
Upcoming patches will expand the qos domain struct and allow it to be
shared between eswitches. For now, qos domains are private to each esw
so there's only an extra indirection.

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