net/mlx4: refactor RSS parent queue allocation

net/mlx4: refactor RSS parent queue allocation

A special "parent" queue must be allocated in addition to a group of
standard Rx queues for RSS to work. This is done automatically outside of
isolated mode by the PMD when applications request several Rx queues.

Since each configured flow rule with the RSS action may target a different
set of queues, the PMD must have the ability to dynamically allocate
several parent queues, one per RSS group.

If isolated mode was requested the default RSS parent queue isn't created
in this case.

Refactor RSS parent queue allocations (currently limited to a single
parent) in preparation for flow API RSS action support.

Signed-off-by: Vasily Philipov <vasilyf@mellanox.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>