Fedora 28: Fix misc bounds check compiler warnings

Fix a bunch of truncation compiler warnings that show up
on Fedora 28 (GCC 8.0.1).

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #7368
Closes #7826
Closes #7830