OpenZFS 7729 - libzfs_core`lzc_rollback() leaks result nvl

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov mail@gmelikov.ru

OpenZFS-issue: https://www.illumos.org/issues/7729
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/882877e
Closes #5620