blk-mq-pci: add a fallback when pci_irq_get_affinity returns NULL

blk-mq-pci: add a fallback when pci_irq_get_affinity returns NULL

While pci_irq_get_affinity should never fail for SMP kernel that
implement the affinity mapping, it will always return NULL in the
UP case, so provide a fallback mapping of all queues to CPU 0 in
that case.

Signed-off-by: Christoph Hellwig <[email protected]>
Cc: [email protected]
Reviewed-by: Omar Sandoval <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>