8046 Let calloc() do the multiplication in libzfs_fru_refresh

https://github.com/illumos/illumos-gate/commit/5697e03e6e3e2697f56ae341c7c8ce79680d6a2e

https://www.illumos.org/issues/8046

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Pedro Giffuni <pfg@freebsd.org>