others/mnt-ext-dev: Force mke2fs to create a filesystem

It's required because we specify a file instead of a block device.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/others/mnt-ext-dev/run.sh

@@ -6,7 +6,7 @@ python ../../zdtm.py run -t zdtm/static/env00 --iter 0 -f ns
 
 truncate -s 0 zdtm.loop
 truncate -s 50M zdtm.loop
-mkfs.ext4 zdtm.loop
+mkfs.ext4 -F zdtm.loop
 dev=`losetup --find --show zdtm.loop`
 mkdir -p ../../dev
 cp -ap $dev ../../dev