MFC r348566: MFC r348554: 9688 aggsum_fini leaks memory

illumos/illumos-gate@29bf2d68bef208274f5a54a14cc80c4a8cb76f53

Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed by: Jorgen Lundman <lundman@lundman.net>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Paul Dagnelie <pcd@delphix.com>