perf_counter: more barrier in blank weak function

Impact: fix panic possible panic

Some versions of GCC inline the weak global function if it's empty.
Add a barrier() to work it around.
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_counter.c

@@ -45,8 +45,8 @@ hw_perf_counter_init(struct perf_counter *counter)
 }
 
 u64 __weak hw_perf_save_disable(void)		{ return 0; }
-void __weak hw_perf_restore(u64 ctrl)		{ }
-void __weak hw_perf_counter_setup(void)		{ }
+void __weak hw_perf_restore(u64 ctrl)		{ barrier(); }
+void __weak hw_perf_counter_setup(void)		{ barrier(); }
 
 static void
 list_add_counter(struct perf_counter *counter, struct perf_counter_context *ctx)