Fix error message when zfs module are already unloaded

Using zfs-sh -u on linux will fail with inaccurate message when the
zfs modules are already unloaded.  Deal with the case where a module 
is already unloaded; its USE_COUNT will be the empty string

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Adam Moss <c@yotes.com>
Closes #11627

scripts/zfs.sh

@@ -198,7 +198,7 @@ unload_modules_linux() {
 
 		if [ "$USE_COUNT" = "0" ] ; then
 			unload_module_linux "$KMOD" || return 1
-		else
+		elif [ "$USE_COUNT" != "" ] ; then
 			echo "Module ${NAME} is still in use!"
 			return 1
 		fi