vdpa/mlx5: fix polling threads scheduling

vdpa/mlx5: fix polling threads scheduling

When the event mode is with 0 fixed delay, the polling-thread will never
give-up CPU.

So, when multi-polling-threads are active, the context-switch between
them will be managed by the system which may affect latency according to
the time-out decided by the system.

In order to fix multi-devices polling thread scheduling, this patch
forces rescheduling for each CQ poll iteration.

Move the polling thread to SCHED_RR mode with maximum priority to
complete the fairness.

Fixes: 6956a48cabbb ("vdpa/mlx5: set polling mode default delay to zero")
Signed-off-by: Matan Azrad <matan@nvidia.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Xueming Li <xuemingl@nvidia.com>