btrfs: use reada direction enum instead of constant value in populate_free_space_tree

populate_free_space_tree calls function btrfs_search_slot_for_read with
parameter int find_higher = 1, it means that, if no exact match is
found, then use the next higher item.  So in function
populate_free_space_tree, use READA_FORWARD to read forward ahead.

This also changes the value from READA_BACK to READA_FORWARD, since
according to the logic, it should reada_for_search forward, not
backward.
Signed-off-by: Gu JinXiang <gujx@cn.fujitsu.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/free-space-tree.c

@@ -1071,7 +1071,7 @@ static int populate_free_space_tree(struct btrfs_trans_handle *trans,
 	path = btrfs_alloc_path();
 	if (!path)
 		return -ENOMEM;
-	path->reada = 1;
+	path->reada = READA_FORWARD;
 
 	path2 = btrfs_alloc_path();
 	if (!path2) {