crush: fix tree node weight lookup

(cherry picked from commit f671d4cd9b36691ac4ef42cde44c1b7a84e13631)

Fix the node weight lookup for tree buckets by using a correct accessor.

Reflects ceph.git commit d287ade5bcbdca82a3aef145b92924cf1e856733.

Reviewed-by: Alex Elder <elder@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>