MFV r247845:
Import ZFS bpobj bugfix from vendor.

Illumos ZFS issues:
  3603 panic from bpobj_enqueue_subobj()
  3604 zdb should print bpobjs more verbosely

References:
  https://www.illumos.org/issues/3603
  https://www.illumos.org/issues/3604

MFC after:	1 week