OpenZFS 6762 - POSIX write should imply DELETE_CHILD on directories

• and some additional considerations

Authored by: Kevin Crowe <kevin.crowe@nexenta.com>
Reviewed by: Gordon Ross <gwr@nexenta.com>
Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Richard Lowe <richlowe@richlowe.net>
Ported-by: Paul B. Henson <henson@acm.org>

OpenZFS-issue: https://www.illumos.org/issues/6762
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/1eb4e906ec
Closes #10266