LU-3656 tests: use full path to directio

LU-3656 tests: use full path to directio

The sanity-benchmark iozone directio test is skipped if started not
from lustre/tests directory because of related path used.
Patch fixes this test to use DIRECTIO instead of ./directio

Signed-off-by: Elena Gryaznova <elena_gryaznova@xyratex.com>
Xyratex-bug-id: MRP-1170
Change-Id: I4dec24b38300df8a14fb8f74f84b4f91c0cde910
Reviewed-on: http://review.whamcloud.com/7173
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>