genirq/matrix: Deal with the sillyness of for_each_cpu() on UP

Most of the CPU mask operations behave the same way, but for_each_cpu() and
it's variants ignore the cpumask argument and claim that CPU0 is always in
the mask. This is historical, inconsistent and annoying behaviour.

The matrix allocator uses for_each_cpu() and can be called on UP with an
empty cpumask. The calling code does not expect that this succeeds but
until commit e027fffff799 ("x86/irq: Unbreak interrupt affinity setting")
this went unnoticed. That commit added a WARN_ON() to catch cases which
move an interrupt from one vector to another on the same CPU. The warning
triggers on UP.

Add a check for the cpumask being empty to prevent this.

Fixes: 2f75d9e1c905 ("genirq: Implement bitmap matrix allocator")
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: [email protected]

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