LU-202 fix the test_23 of sanity-quota.sh

LU-202 fix the test_23 of sanity-quota.sh

test_23() uses a constant value(120000 = ~120M) to check if the
system has enough space to finish the test, so in the large OSTCOUNT
testing environment, the test will fail for short of disk space.

Changes of this patch are:
- Use a reasonable value to check free space instead of magic 120000;
- Specify the stripe index of test file for test_9 and test_23;

Issue:
Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: If7cd01b7ce9dc025a0f1b8b70fa9214d70182a86
Reviewed-on: http://review.whamcloud.com/434
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>