Add zfs.sh -r flag to reload modules

zfs.sh already can load and unload, so why not both?

This is convenient when developing changes to the module and you want
to rapidly make some changes, rebuild the module, reload the module,
and test the changes.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #12450