selftests/mount_settattr: ensure that ext4 filesystem can be created

Filesystem too small for a journal
mount: /mnt/D/: mount failed: Operation not permitted.
mount_setattr_test.c:1076:idmap_mount_tree_invalid:Expected system("mount -o loop -t ext4 /mnt/C/ext4.img /mnt/D/") (256) == 0 (0)

Signed-off-by: Christian Brauner <brauner@kernel.org>