KVM: x86/mmu: Revert "Revert "KVM: MMU: reclaim the zapped-obsolete page first""

Now that the fast invalidate mechanism has been reintroduced, restore
the performance tweaks for fast invalidation that existed prior to its
removal.

Paraphrashing the original changelog:

  Introduce a per-VM list to track obsolete shadow pages, i.e. pages
  which have been deleted from the mmu cache but haven't yet been freed.
  When page reclaiming is needed, zap/free the deleted pages first.

This reverts commit 52d5dedc.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -865,6 +865,7 @@ struct kvm_arch {
 	 * Hash table of struct kvm_mmu_page.
 	 */
 	struct list_head active_mmu_pages;
+	struct list_head zapped_obsolete_pages;
 	struct kvm_page_track_notifier_node mmu_sp_tracker;
 	struct kvm_page_track_notifier_head track_notifier_head;