MFC r348559: MFV r348537: 8601 memory leak in get_special_prop()

illumos/illumos-gate@e19b450bec203d8be04447ea476d7a86b36d63a1

Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed by: Sara Hartse <sara.hartse@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: John Gallagher <john.gallagher@delphix.com>