btrfs: add barrier for waitqueue_active in clear_btree_io_tree

waitqueue_active should be preceded by a barrier, in this function we
don't need to call it all the time.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/transaction.c

@@ -82,6 +82,12 @@ void btrfs_put_transaction(struct btrfs_transaction *transaction)
 static void clear_btree_io_tree(struct extent_io_tree *tree)
 {
 	spin_lock(&tree->lock);
+	/*
+	 * Do a single barrier for the waitqueue_active check here, the state
+	 * of the waitqueue should not change once clear_btree_io_tree is
+	 * called.
+	 */
+	smp_mb();
 	while (!RB_EMPTY_ROOT(&tree->state)) {
 		struct rb_node *node;
 		struct extent_state *state;