Use WARN_ON_ONCE for missing X86_FEATURE_NRIPS

The cpu feature flags are not ever going to change, so warning
everytime can cause a lot of kernel log spam
(in our case more than 10GB/hour).

The warning seems to only occur when nested virtualization is
enabled, so it's probably triggered by a KVM bug.  This is a
sensible and safe change anyway, and the KVM bug fix might not
be suitable for stable releases anyway.

Cc: stable@vger.kernel.org
Signed-off-by: Dirk Mueller <dmueller@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>