rbd: prevent kernel stack blow up on rbd map

commit 6d69bb536bac0d403d83db1ca841444981b280cd upstream.

Mapping an image with a long parent chain (e.g. image foo, whose parent
is bar, whose parent is baz, etc) currently leads to a kernel stack
overflow, due to the following recursion in the reply path:

  rbd_osd_req_callback()
    rbd_obj_request_complete()
      rbd_img_obj_callback()
        rbd_img_parent_read_callback()
          rbd_obj_request_complete()
            ...

Limit the parent chain to 16 images, which is ~5K worth of stack.  When
the above recursion is eliminated, this limit can be lifted.

Fixes: http://tracker.ceph.com/issues/12538



Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Josh Durgin <jdurgin@redhat.com>
[idryomov@gmail.com: backport to 3.10: rbd_dev->opts, context]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>