Fix opening oat files that are out of date.

Make sure we're not using an old MAP_PRIVATE mapping of an
OatFile after a forked process modifies the underlying file.

Change-Id: I5c6caaf34272c805e40e95ee690dd948d7406751
diff --git a/runtime/class_linker.h b/runtime/class_linker.h
index d468e1e..8722de3 100644
--- a/runtime/class_linker.h
+++ b/runtime/class_linker.h
@@ -251,7 +251,8 @@
   // Generate an oat file from a dex file
   bool GenerateOatFile(const char* dex_filename,
                        int oat_fd,
-                       const char* oat_cache_filename);
+                       const char* oat_cache_filename,
+                       std::string* error_msg);
       LOCKS_EXCLUDED(Locks::mutator_lock_);
 
   const OatFile* FindOatFileFromOatLocation(const std::string& location,