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>