genirq/affinity: Handle pre/post vectors in irq_calc_affinity_vectors()

Only calculate the affinity for the main I/O vectors, and skip the pre or
post vectors specified by struct irq_affinity.

Also remove the irq_affinity cpumask argument that has never been used.  If
we ever need it in the future we can pass it through struct irq_affinity.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: linux-pci@vger.kernel.org
Link: http://lkml.kernel.org/r/1478654107-7384-3-git-send-email-hch@lst.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>