KVM: nVMX: Drop STACK_FRAME_NON_STANDARD from nested_vmx_check_vmentry_hw()

...as it doesn't technically actually do anything non-standard with the
stack even though it modifies RSP in a weird way.  E.g. RSP is loaded
with VMCS.HOST_RSP if the VM-Enter gets far enough to trigger VM-Exit,
but it's simply reloaded with the current value.

Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>