KVM: X86: Zap the invalid list after remote tlb flushing

In mmu_sync_children(), it can zap the invalid list after remote tlb flushing.
Emptifying the invalid list ASAP might help reduce a remote tlb flushing
in some cases.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210918005636.3675-8-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>