x86/access: Stop pretending the test is SMP friendly

Remove SMP "support", the test is not remotely SMP friendly.  It can
barely survive one CPU modifying page tables, two would be pure carnage.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>