KVM: selftests: Use vm_get_page_table_entry() in addr_arch_gva2gpa()

Use vm_get_page_table_entry() in addr_arch_gva2gpa() to get the leaf PTE
instead of manually walking page tables.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20221006004512.666529-7-seanjc@google.com