Illumos 3604 - zdb should print bpobjs more verbosely (fix zdb hang)

3604 zdb should print bpobjs more verbosely (fix zdb hang)
References:

https://github.com/illumos/illumos-gate/commit/7706186
https://www.illumos.org/issues/3604
https://lists.freebsd.org/pipermail/svn-src-vendor/2015-August/002411.html
https://lists.freebsd.org/pipermail/svn-src-head/2015-August/075195.html

Porting notes:
In ZoL "5810 zdb should print details of bpobj" was merged
prior to this change so it must be applied to the new location.

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>