Add zfs.sh module unload error message

If modules fail to unload because of outstanding users, don't 
consider this a success.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Adam Moss <c@yotes.com>
Closes #11042

scripts/zfs.sh

@@ -198,6 +198,9 @@ unload_modules_linux() {
 
 		if [ "$USE_COUNT" = "0" ] ; then
 			unload_module_linux "$KMOD" || return 1
+		else
+			echo "Module ${NAME} is still in use!"
+			return 1
 		fi
 	done