Don't recursively take ClassLinker::dex_lock_.

Fix for Bug: 9285048.

Change-Id: I4bdefdc7e1de2eec6488e8d629147d9699f4d099
diff --git a/src/base/mutex.h b/src/base/mutex.h
index d6ae3dc..e7d2e97 100644
--- a/src/base/mutex.h
+++ b/src/base/mutex.h
@@ -247,7 +247,7 @@
   // Assert the current thread doesn't have exclusive access to the ReaderWriterMutex.
   void AssertNotExclusiveHeld(const Thread* self) {
     if (kDebugLocking) {
-      CHECK(!IsExclusiveHeld(self));
+      CHECK(!IsExclusiveHeld(self)) << *this;
     }
   }
   void AssertNotWriterHeld(const Thread* self) { AssertNotExclusiveHeld(self); }