KVM: x86/mmu: Voluntarily reschedule as needed when zapping MMIO sptes

Call cond_resched_lock() when zapping MMIO to reschedule if needed or to
release and reacquire mmu_lock in case of contention.  There is no need
to flush or zap when temporarily dropping mmu_lock as zapping MMIO sptes
is done when holding the memslots lock and with the "update in-progress"
bit set in the memslots generation, which disables MMIO spte caching.
The walk does need to be restarted if mmu_lock is dropped as the active
pages list may be modified.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu.c

@@ -5954,7 +5954,8 @@ static void kvm_mmu_zap_mmio_sptes(struct kvm *kvm)
 	list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
 		if (!sp->mmio_cached)
 			continue;
-		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
+		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list) ||
+		    cond_resched_lock(&kvm->mmu_lock))
 			goto restart;
 	}