btrfs: add comments to barriers before waitqueue_active

Reduce number of undocumented barriers out there.

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -861,6 +861,9 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 	atomic_dec(&cur_trans->num_writers);
 	extwriter_counter_dec(cur_trans, trans->type);
 
+	/*
+	 * Make sure counter is updated before we wake up waiters.
+	 */
 	smp_mb();
 	if (waitqueue_active(&cur_trans->writer_wait))
 		wake_up(&cur_trans->writer_wait);