KVM: selftests: Use vcpu_arch_put_guest() in mmu_stress_test

Use vcpu_arch_put_guest() to write memory from the guest in
mmu_stress_test as an easy way to provide a bit of extra coverage.

Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/mmu_stress_test.c

@@ -23,7 +23,7 @@ static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
 
 	for (;;) {
 		for (gpa = start_gpa; gpa < end_gpa; gpa += stride)
-			*((volatile uint64_t *)gpa) = gpa;
+			vcpu_arch_put_guest(*((volatile uint64_t *)gpa), gpa);
 		GUEST_SYNC(0);
 	}
 }