7990 libzfs: snapspec_cb() does not need to call zfs_strdup()

illumos/illumos-gate@d8584ba6fb7a5e46da1725845b99ae5fab5a4baf
https://github.com/illumos/illumos-gate/commit/d8584ba6fb7a5e46da1725845b99ae5fab5a4baf

https://www.illumos.org/issues/7990

The snapspec_cb() callback function in libzfs does not need to call zfs_strdup().

Reviewed by: Yuri Pankov <yuri.pankov@gmail.com>
Reviewed by: Toomas Soome <tsoome@me.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Author: Marcel Telka <marcel@telka.sk>