MFV 286602: 5810 zdb should print details of bpobj

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Will Andrews <will@freebsd.org>
Reviewed by: Simon Klinkert <simon.klinkert@gmail.com>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Matthew Ahrens <mahrens@delphix.com>

illumos/illumos-gate@732885fca09e11183dd0ea69aaaab5588fb7dbff