scripts: zfs.sh: explicitly ignore unloaded modules when unloading

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

scripts/zfs.sh

@@ -164,7 +164,7 @@ unload_modules_freebsd() {
 unload_modules_linux() {
 	NAME="${KMOD_ZFS##*/}"
 	NAME="${NAME%.ko}"
-	$DELMOD "$NAME" || return
+	! [ -d "/sys/module/$NAME" ] || $DELMOD "$NAME" || return
 
 	if [ "$VERBOSE" = "yes" ]; then
 		echo "Successfully unloaded ZFS module stack"