runltp: try to create ext3 loop device if creating ext4 failed

Try to create ext3 loop device if creating ext4 failed since
some old system can't support ext4 file system or the root
file system still use ext3 file system.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>