KVM: MMU: reschedule during shadow teardown

Shadows for large guests can take a long time to tear down, so reschedule
occasionally to avoid softlockup warnings.

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 7246b60..c2fd6a4 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1858,6 +1858,7 @@
 		sp = container_of(vcpu->kvm->arch.active_mmu_pages.next,
 				  struct kvm_mmu_page, link);
 		kvm_mmu_zap_page(vcpu->kvm, sp);
+		cond_resched();
 	}
 	free_page((unsigned long)vcpu->arch.mmu.pae_root);
 }