KVM: MMU: fix shrinking page from the empty mmu

Fix:

 [ 3190.059226] BUG: unable to handle kernel NULL pointer dereference at           (null)
 [ 3190.062224] IP: [<ffffffffa02aac66>] mmu_page_zap_pte+0x10/0xa7 [kvm]
 [ 3190.063760] PGD 104f50067 PUD 112bea067 PMD 0
 [ 3190.065309] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
 [ 3190.066860] CPU 1
[ ...... ]
 [ 3190.109629] Call Trace:
 [ 3190.111342]  [<ffffffffa02aada6>] kvm_mmu_prepare_zap_page+0xa9/0x1fc [kvm]
 [ 3190.113091]  [<ffffffffa02ab2f5>] mmu_shrink+0x11f/0x1f3 [kvm]
 [ 3190.114844]  [<ffffffffa02ab25d>] ? mmu_shrink+0x87/0x1f3 [kvm]
 [ 3190.116598]  [<ffffffff81150c9d>] ? prune_super+0x142/0x154
 [ 3190.118333]  [<ffffffff8110a4f4>] ? shrink_slab+0x39/0x31e
 [ 3190.120043]  [<ffffffff8110a687>] shrink_slab+0x1cc/0x31e
 [ 3190.121718]  [<ffffffff8110ca1d>] do_try_to_free_pages

This is caused by shrinking page from the empty mmu, although we have
checked n_used_mmu_pages, it is useless since the check is out of mmu-lock
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/mmu.c

@@ -3934,6 +3934,9 @@ static void kvm_mmu_remove_some_alloc_mmu_pages(struct kvm *kvm,
 {
 	struct kvm_mmu_page *page;
 
+	if (list_empty(&kvm->arch.active_mmu_pages))
+		return;
+
 	page = container_of(kvm->arch.active_mmu_pages.prev,
 			    struct kvm_mmu_page, link);
 	kvm_mmu_prepare_zap_page(kvm, page, invalid_list);