Add compatibility for busybox mktemp

Busybox's mktemp requires at least six X's in the template, causing
the current sed --in-place check to fail because the file does not
exist. This change adds additional X's to mktemp templates that do
not already have at least six X's in them.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Quentin Zdanis <zdanisq@gmail.com>
Closes #11269