6873 zfs_destroy_snaps_nvl leaks errlist

illumos/illumos-gate@4cde22c29999ffb907ca39d2ebd512812f7e5168
https://github.com/illumos/illumos-gate/commit/4cde22c29999ffb907ca39d2ebd512812f7e5168

https://www.illumos.org/issues/6873

lzc_destroy_snaps() returns an nvlist in errlist.
zfs_destroy_snaps_nvl() should nvlist_free() it before returning.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Chris Williamson <chris.williamson@delphix.com>