dm bufio: update comment in dm-bufio.c

* Hashtable has been replaced by rbtree to manage buffers.
  Update the comment.
* Fix typo in the comment for dm_bufio_issue_flush

Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>