8590 memory leak in dsl_destroy_snapshots_nvl()

illumos/illumos-gate@e6ab4525d156c82445c116ecf6b2b874d5e9009d
https://github.com/illumos/illumos-gate/commit/e6ab4525d156c82445c116ecf6b2b874d5e9009d

https://www.illumos.org/issues/8590

In dsl_destroy_snapshots_nvl(), "snaps_normalized" is not freed after it is
added to "arg".

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Steve Gonczi <steve.gonczi@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>