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

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

Authored by: Marcel Telka <marcel@telka.sk>
Reviewed by: Yuri Pankov <yuri.pankov@gmail.com>
Reviewed by: Toomas Soome <tsoome@me.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/7990
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/d8584ba
Closes #5939