Do not allow populating the same page twice with startup data.  In the
case of SEV-SNP, for example, the firmware does not allow it anyway,
since the launch-update operation is only possible on pages that are
still shared in the RMP.

Even if it worked, kvm_gmem_populate()'s callback is meant to have side
effects such as updating launch measurements, and updating the same
page twice is unlikely to have the desired results.

Races between calls to the ioctl are not possible because
kvm_gmem_populate() holds slots_lock and the VM should not be running.
But again, even if this worked on other confidential computing technology,
it doesn't matter to guest_memfd.c whether this is something fishy
such as missing synchronization in userspace, or rather something
intentional.  One of the racers wins, and the page is initialized by
either kvm_gmem_prepare_folio() or kvm_gmem_populate().

Anyway, out of paranoia, adjust sev_gmem_post_populate() anyway to use
the same errno that kvm_gmem_populate() is using.
Reviewed-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>