MFV r316932: 6280 libzfs: unshare_one() could fail with EZFS_SHARENFSFAILED

MFV r316932: 6280 libzfs: unshare_one() could fail with EZFS_SHARENFSFAILED

illumos/illumos-gate@d1672efb6feac57c42788e27f739dfa3c4f3baf7
https://github.com/illumos/illumos-gate/commit/d1672efb6feac57c42788e27f739dfa3c4f3baf7

https://www.illumos.org/issues/6280
  The unshare_one() in libzfs could fail with EZFS_SHARENFSFAILED at line 834
  here:
  831    /* make sure libshare initialized */
  832    if ((err = zfs_init_libshare(hdl, SA_INIT_SHARE_API)) != SA_OK) {
  833        free(mntpt);    /* don't need the copy anymore */
  834        return (zfs_error_fmt(hdl, EZFS_SHARENFSFAILED,
  835            dgettext(TEXT_DOMAIN, "cannot unshare '%s': %s"),
  836            name, _sa_errorstr(err)));
  837    }
  The correct error should be EZFS_UNSHARENFSFAILED instead.

Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Marcel Telka <marcel.telka@nexenta.com>

MFC after:	4 weeks