sched/topology: Fix sched_numa_find_nth_cpu() in CPU-less case

When the node provided by user is CPU-less, corresponding record in
sched_domains_numa_masks is not set. Trying to dereference it in the
following code leads to kernel crash.

To avoid it, start searching from the nearest node with CPUs.

Fixes: cd7f55359c90 ("sched: add sched_numa_find_nth_cpu()")
Reported-by: Yicong Yang <[email protected]>
Reported-by: Guenter Roeck <[email protected]>
Signed-off-by: Yury Norov <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Yicong Yang <[email protected]>
Cc: Mel Gorman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Closes: https://lore.kernel.org/lkml/CAAH8bW8C5humYnfpW3y5ypwx0E-09A3QxFE1JFzR66v+mO4XfA@mail.gmail.com/T/
Closes: https://lore.kernel.org/lkml/ZMHSNQfv39HN068m@yury-ThinkPad/T/#mf6431cb0b7f6f05193c41adeee444bc95bf2b1c4

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