[JVMCI] c2v_readFieldValue use long instead of jlong for the offset parameter

GivThe following should be using jlong and not long for the type of the displacement:
C2V_VMENTRY_NULL(jobject, readFieldValue, (JNIEnv* env, jobject, jobject object, jobject expected_type, long displacement, jboolean is_volatile, jobject kind_object))