generic/391: add _require_odirect

generic/391: add _require_odirect

On filesystems that do not support O_DIRECT, the test should skip
itself with

 generic/391	 [not run] O_DIRECT is not supported

instead of failing with

 generic/391	 - output mismatch ...
     ...
     +open: Invalid argument
      Silence is golden
     ...

This patch make it do that by adding a call to _require_odirect.
Fixes https://github.com/rfjakob/gocryptfs/issues/244 .

Signed-off-by: Jakob Unterwurzacher <jakobunt@gmail.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>