genirq/affinity: Rename irq_build_affinity_masks as group_cpus_evenly

Map irq vector into group, which allows to abstract the algorithm for
a generic use case outside of the interrupt core.

Rename irq_build_affinity_masks as group_cpus_evenly, so the API can be
reused for blk-mq to make default queue mapping even though irq vectors
aren't involved.

No functional change, just rename vector as group.

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: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20221227022905.352674-5-ming.lei@redhat.com