Btrfs: fix confusing worker helper info in stacktrace

We've seen the following backtrace stack in ftrace or dmesg log,

  kworker/u16:10-4244  [000] 241942.480955: function:             btrfs_put_ordered_extent
  kworker/u16:10-4244  [000] 241942.480956: kernel_stack:         <stack trace>
=> finish_ordered_fn (ffffffffa0384475)
=> btrfs_scrubparity_helper (ffffffffa03ca577)        <-----"incorrect"
=> btrfs_freespace_write_helper (ffffffffa03ca98e)    <-----"correct"
=> process_one_work (ffffffff81117b2f)
=> worker_thread (ffffffff81118c2a)
=> kthread (ffffffff81121de0)
=> ret_from_fork (ffffffff81d7087a)

btrfs_freespace_write_helper is actually calling normal_worker_helper
instead of btrfs_scrubparity_helper, so somehow kernel has parsed the
incorrect function address while unwinding the stack,
btrfs_scrubparity_helper really shouldn't be shown up.

It's caused by compiler doing inline for our helper function, adding a
noinline tag can fix that.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ use noinline_for_stack ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/async-thread.c

@@ -67,7 +67,7 @@ struct btrfs_workqueue {
 static void normal_work_helper(struct btrfs_work *work);
 
 #define BTRFS_WORK_HELPER(name)					\
-void btrfs_##name(struct work_struct *arg)				\
+noinline_for_stack void btrfs_##name(struct work_struct *arg)		\
 {									\
 	struct btrfs_work *work = container_of(arg, struct btrfs_work,	\
 					       normal_work);		\