KVM/x86: Call smp_wmb() before increasing tlbs_dirty

Update spte before increasing tlbs_dirty to make sure no tlb flush
in lost after spte is zapped. This pairs with the barrier in the
kvm_flush_remote_tlbs().

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>