Fix two read barrier bugs introduced in CL 97882.

Bug: 12687968
Change-Id: Ib28da4c33977cc58b09913ef5b738dec75365714
diff --git a/runtime/class_linker.cc b/runtime/class_linker.cc
index 1436810..ea02ab1 100644
--- a/runtime/class_linker.cc
+++ b/runtime/class_linker.cc
@@ -2563,8 +2563,9 @@
 
   for (auto it = class_table_.lower_bound(hash), end = class_table_.end(); it != end && it->first == hash;
        ++it) {
-    mirror::Class* entry = it->second;
-    if (entry == existing) {
+    mirror::Class** root = &it->second;
+    mirror::Class* klass = ReadBarrier::BarrierForRoot<mirror::Class, kWithReadBarrier>(root);
+    if (klass == existing) {
       class_table_.erase(it);
       break;
     }