net/mlx5: Introduce mlx5_cpumask_default_spread

For better code readability in the completion IRQ request code, define
the cpu lookup per completion vector logic in a separate function.

The new method mlx5_cpumask_default_spread() given a vector index 'n'
will return the 'nth' cpu. This new method will be used also in the next
patch.

Signed-off-by: Maher Sanalla <msanalla@nvidia.com>
Reviewed-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>