blk-mq: create correct map for fallback case

The fallback code in blk_mq_map_hw_queues is original from
blk_mq_pci_map_queues and was added to handle the case where
pci_irq_get_affinity will return NULL for !SMP configuration.

blk_mq_map_hw_queues replaces besides blk_mq_pci_map_queues also
blk_mq_virtio_map_queues which used to use blk_mq_map_queues for the
fallback.

It's possible to use blk_mq_map_queues for both cases though.
blk_mq_map_queues creates the same map as blk_mq_clear_mq_map for !SMP
that is CPU 0 will be mapped to hctx 0.

The WARN_ON_ONCE has to be dropped for virtio as the fallback is also
taken for certain configuration on default. Though there is still a
WARN_ON_ONCE check in lib/group_cpus.c:

       WARN_ON(nr_present + nr_others < numgrps);

which will trigger if the caller tries to create more hardware queues
than CPUs. It tests the same as the WARN_ON_ONCE in
blk_mq_pci_map_queues did.

Fixes: a5665c3d15 ("virtio: blk/scsi: replace blk_mq_virtio_map_queues with blk_mq_map_hw_queues")
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Closes: https://lore.kernel.org/all/20250122093020.6e8a4e5b@gandalf.local.home/
Signed-off-by: Daniel Wagner <wagi@kernel.org>
Link: https://lore.kernel.org/r/20250123-fix-blk_mq_map_hw_queues-v1-1-08dbd01f2c39@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-cpumap.c

@@ -87,7 +87,6 @@ void blk_mq_map_hw_queues(struct blk_mq_queue_map *qmap,
 	return;
 
 fallback:
-	WARN_ON_ONCE(qmap->nr_queues > 1);
+	blk_mq_map_queues(qmap);
-	blk_mq_clear_mq_map(qmap);
 }
 EXPORT_SYMBOL_GPL(blk_mq_map_hw_queues);