8003635: NPG: AsynchGetCallTrace broken by Method* virtual call

Make metaspace::contains be lock free and used to see if something is in metaspace, also compare Method* with vtbl pointer.

Reviewed-by: dholmes, sspitsyn, dcubed, jmasa

hotspot/src/share/vm/oops/method.hpp

@@ -169,7 +169,8 @@ class Method : public Metadata {
                           ConstMethod::MethodType method_type,
                           TRAPS);
 
-  Method() { assert(DumpSharedSpaces || UseSharedSpaces, "only for CDS"); }
+  // CDS and vtbl checking can create an empty Method to get vtbl pointer.
+  Method(){}
 
   // The Method vtable is restored by this call when the Method is in the
   // shared archive.  See patch_klass_vtables() in metaspaceShared.cpp for
@@ -812,6 +813,9 @@ class Method : public Metadata {
 
   const char* internal_name() const { return "{method}"; }
 
+  // Check for valid method pointer
+  bool is_valid_method() const;
+
   // Verify
   void verify() { verify_on(tty); }
   void verify_on(outputStream* st);