btrfs: reada: move reada_extent_put to place after __readahead_hook()

We can't release reada_extent earlier than __readahead_hook(), because
__readahead_hook() still need to use it, it is necessary to hode a refcnt
to avoid it be freed.

Actually it is not a problem after my patch named:
  Avoid many times of empty loop
It make reada_extent in above line include at least one reada_extctl,
which keeps additional one refcnt for reada_extent.

But we still need this patch to make the code in pretty logic.
Signed-off-by: NZhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/reada.c

@@ -717,8 +717,6 @@ static int reada_start_machine_dev(struct btrfs_fs_info *fs_info,
 	}
 	logical = re->logical;
 
-	reada_extent_put(fs_info, re);
-
 	atomic_inc(&dev->reada_in_flight);
 	ret = reada_tree_block_flagged(fs_info->extent_root, logical,
 			mirror_num, &eb);
@@ -730,6 +728,8 @@ static int reada_start_machine_dev(struct btrfs_fs_info *fs_info,
 	if (eb)
 		free_extent_buffer(eb);
 
+	reada_extent_put(fs_info, re);
+
 	return 1;
 
 }