KVM: MMU: Fix rmap_write_protect() hugepage iteration bug

rmap_next() does not work correctly after rmap_remove(), as it expects
the rmap chains not to change during iteration.  Fix (for now) by restarting
iteration from the beginning.
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/mmu.c

@@ -640,6 +640,7 @@ static void rmap_write_protect(struct kvm *kvm, u64 gfn)
 			rmap_remove(kvm, spte);
 			--kvm->stat.lpages;
 			set_shadow_pte(spte, shadow_trap_nonpresent_pte);
+			spte = NULL;
 			write_protected = 1;
 		}
 		spte = rmap_next(kvm, rmapp, spte);