OpenZFS 7027 - zfs_written_property_001_pos makes unreasonable assumptions about...

OpenZFS 7027 - zfs_written_property_001_pos makes unreasonable assumptions about metadata space usage

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Approved by: Gordon Ross <gordon.ross@nexenta.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/7027
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/48cb8b9
Issue #2441
Closes #5778