scripts: zfs.sh: explicitly unload all modules via rmmod

modprobe -r only works for depmodded modules, but this also means we
have to re-iterate legacy modules, and in the right order

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13356