KVM: MMU: Topup the mmu memory preallocation caches before emulating an insn

Emulation may cause a shadow pte to be instantiated, which requires
memory resources.  Make sure the caches are filled to avoid an oops.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/mmu.c b/drivers/kvm/mmu.c
index ace3cb8..9be54a5 100644
--- a/drivers/kvm/mmu.c
+++ b/drivers/kvm/mmu.c
@@ -1362,6 +1362,10 @@
 		goto out;
 	}
 
+	r = mmu_topup_memory_caches(vcpu);
+	if (r)
+		goto out;
+
 	er = emulate_instruction(vcpu, vcpu->run, cr2, error_code, 0);
 	mutex_unlock(&vcpu->kvm->lock);