x86: fix small sparse warning

arch/x86/kernel/ds.c:226:9: warning: Using plain integer as NULL pointer
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/ds.c

@@ -223,7 +223,7 @@ int ds_free(void **dsp)
 	if (*dsp)
 		kfree((void *)get_bts_buffer_base(*dsp));
 	kfree(*dsp);
-	*dsp = 0;
+	*dsp = NULL;
 
 	return 0;
 }