KVM: SVM: Don't kill SEV guest if SMAP erratum triggers in usermode

Inject a #GP instead of synthesizing triple fault to try to avoid killing
the guest if emulation of an SEV guest fails due to encountering the SMAP
erratum.  The injected #GP may still be fatal to the guest, e.g. if the
userspace process is providing critical functionality, but KVM should
make every attempt to keep the guest alive.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Message-Id: <20220120010719.711476-10-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>