KVM: selftests: Don't fall over in mmu_stress_test when only one CPU is present

KVM: selftests: Don't fall over in mmu_stress_test when only one CPU is present

Running mmu_stress_test on a system with only one CPU is not a recipe for
success. However, there's no clear-cut reason why it absolutely
shouldn't work, so the test shouldn't completely reject such a platform.

At present, the *3/4 calculation will return zero on these platforms and
the test fails. So, instead just skip that calculation.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Link: https://lore.kernel.org/r/20251007-b4-kvm-mmu-stresstest-1proc-v1-1-8c95aa0e30b6@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>