genirq/affinity: Remove the 'firstvec' parameter from irq_build_affinity_masks

The 'firstvec' parameter is always same with the parameter of
'startvec', so use 'startvec' directly inside irq_build_affinity_masks().

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20221227022905.352674-2-ming.lei@redhat.com