zdtm:mnt_ext_dev: output a message if ZDTM_MNT_EXT_DEV is not set

Should be easier to debug if the variable is not passed by accident.

travis-ci: success for series starting with [1/2] zdtm:mnt_ext_auto: avoid segfault if ZDTM_NEWNS is not set
Signed-off-by: Dmitrii Shcherbakov <dshcherbakov@virtuozzo.com>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/static/mnt_ext_dev.c

@@ -29,8 +29,10 @@ int main(int argc, char **argv)
 
 	mkdir(dirname, 0777);
 	loop = getenv("ZDTM_MNT_EXT_DEV");
-	if (loop == NULL)
+	if (loop == NULL) {
+		pr_perror("ZDTM_MNT_EXT_DEV is not set");
 		return 1;
+	}
 
 	if (mount(loop, dirname, "ext4", 0, NULL) == -1) {
 		pr_perror("mount");