LU-7129 tests: fsx with directio

LU-7129 tests: fsx with directio

This change adds O_DIRECT support to the existing fsx in lustre/tests.
It also add sanityn test_16 to excercise this option to fsx.

Upstream Xfstest-commit cad234da750aceb563992d95a0b14379740aedc6

Test-Parameters: trivial testlist=sanityn
Signed-off-by: Vitaly Fertman <vitaly.fertman@seagate.com>
Signed-off-by: Parinay Kondekar <parinay.kondekar@seagate.com>
Change-Id: Ibd6a8dfeff8afaf08eeb3f2f8bddd7cdb688e8cf
Seagate-bug-id: MRP-852
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@seagate.com>
Reviewed-by: Elena V. Gryaznova <elena.gryaznova@seagate.com>
Tested-by: Elena V. Gryaznova <elena.gryaznova@seagate.com>
Reviewed-on: https://review.whamcloud.com/16357
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>