KVM: x86/mmu: Don't do TLB flush when zappings SPTEs in invalid roots

Don't force a TLB flush when zapping SPTEs in invalid roots as vCPUs
can't be actively using invalid roots (zapping SPTEs in invalid roots is
necessary only to ensure KVM doesn't mark a page accessed/dirty after it
is freed by the primary MMU).

Link: https://lore.kernel.org/r/20240111020048.844847-3-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>