kvm: vmx: fix oops with explicit flexpriority=0 option

A function pointer was not NULLed, causing kvm_vcpu_reload_apic_access_page to
go down the wrong path and OOPS when doing put_page(NULL).

This did not happen on old processors, only when setting the module option
explicitly.

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