LU-213 using direct io for quota test to avoid cache problem

LU-213 using direct io for quota test to avoid cache problem

in test_14a of sanity-qutota.sh, the data written was cached in client side,
which causes the test failed, using "mulitop ${TESTFILE} oO_DIRECT" (direct IO)
instead of "dd" to avoid such issue.

Change-Id: I0802c2ea200483c148e4e7f61e2c6d4b34500f01
Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/435
Tested-by: Hudson
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>