OpenZFS 8046 - Let calloc() do the multiplication in libzfs_fru_refresh

OpenZFS 8046 - Let calloc() do the multiplication in libzfs_fru_refresh

Authored by: Pedro Giffuni <pfg@freebsd.org>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/8046
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/3a3c0d5
Closes #5989