genirq/affinity: Add support for allocating interrupt sets

A driver may have a need to allocate multiple sets of MSI/MSI-X interrupts,
and have them appropriately affinitized.

Add support for defining a number of sets in the irq_affinity structure, of
varying sizes, and get each set affinitized correctly across the machine.

[ tglx: Minor changelog tweaks ]

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Cc: linux-block@vger.kernel.org
Link: https://lkml.kernel.org/r/20181102145951.31979-5-ming.lei@redhat.com