OpenZFS 6873 - zfs_destroy_snaps_nvl leaks errlist

OpenZFS 6873 - zfs_destroy_snaps_nvl leaks errlist

Authored by: Chris Williamson <chris.williamson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Ported-by: Denys Rtveliashvili <denys@rtveliashvili.name>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
lzc_destroy_snaps() returns an nvlist in errlist.
zfs_destroy_snaps_nvl() should nvlist_free() it before returning.

OpenZFS-issue: https://www.illumos.org/issues/6873
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/ee06391
Closes #4614