MFC r316904: 7729 libzfs_core`lzc_rollback() leaks result nvl

illumos/illumos-gate@ac428481f96be89add7a1edf43ae47dd71038553
https://github.com/illumos/illumos-gate/commit/ac428481f96be89add7a1edf43ae47dd71038553

https://www.illumos.org/issues/7729

libzfs_core`lzc_rollback() doesn't free the result nvl after lzc_ioctl() call.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Yuri Pankov <yuri.pankov@nexenta.com>

MFC after: 2 weeks