OpenZFS 7801 - add more by-dnode routines (lint)

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/7801
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/f25efb3
Closes #5894