dm bio prison: use rb_entry() rather than container_of()

To make the code clearer, use rb_entry() instead of container_of() to
deal with rbtree.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>