irq: pass first vector to __irq_build_affinity_masks

irq: pass first vector to __irq_build_affinity_masks

No functional change, and prepare for the following patch to
support allocating (and affinitizing) sets of IRQs, in which
each set of IRQ needs whole 2-stage spread, and the 1st vector
should point to the 1st one in this set.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: Hannes Reinecke <hare@suse.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>