MFV r275542:

If a dnode has a spill block and there is an error while accessing
a data block then traverse_dnode() loses information about that error
and returns a status of visiting the spill block.

This issue is discovered by Spectra Logic.

Illumos issue:

5311 traverse_dnode may report success when it should not

Original author: gibbs
MFC after: 2 weeks