KVM/MMU: Simplify __kvm_sync_page() function

Merge check of "sp->role.cr4_pae != !!is_pae(vcpu))" and "vcpu->
arch.mmu.sync_page(vcpu, sp) == 0". kvm_mmu_prepare_zap_page()
is called under both these conditions.

Signed-off-by: Lan Tianyu <Tianyu.Lan@microsoft.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>