KVM: x86/mmu: Use IS_ENABLED() to avoid RETPOLINE for TDP page faults

Use IS_ENABLED() instead of an #ifdef to activate the anti-RETPOLINE fast
path for TDP page faults.  The generated code is identical, and the #ifdef
makes it dangerously difficult to extend the logic (guess who forgot to
add an "else" inside the #ifdef and ran through the page fault handler
twice).

No functional or binary change intented.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220423034752.1161007-9-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>