MFV r323523: 8331 zfs_unshare returns wrong error code for smb unshare failure

illumos/illumos-gate@4f4378cc54b7deec3a35c529dc397dbdc325b4bb
https://github.com/illumos/illumos-gate/commit/4f4378cc54b7deec3a35c529dc397dbdc325b4bb

https://www.illumos.org/issues/8331

zfs_unshare returns EZFS_UNSHARENFSFAILED on error for all share types.

Reviewed by: Marcel Telka <marcel@telka.sk>
Reviewed by: Toomas Soome <tsoome@me.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Andrew Stormont <astormont@racktopsystems.com>

MFC after: 4 weeks