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: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>