fiemap-tester: convert to use O_DIRECT

fiemap-tester: convert to use O_DIRECT

We want to test fiemap, so we want to lay out a file exactly a certain way.  XFS
tries to be smart and will allocate data in a hole if the write pattern is

[data][hole][data]

where the hole is small enough.  This screws with fiemap-tester sometimes, so
make it easy and just do O_DIRECT so that we get the layout we want and can get
back to testing fiemap.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>