xfstests: allow override of XFS_IOC_DIOINFO

xfstests: allow override of XFS_IOC_DIOINFO

This change allows xfstests runs to simulate apps
which don't bother to call XFS_IOC_DIOINFO, and simply
issue DIO in sizes and alignments of its own choosing.

So i.e.:

# export XFS_DIO_MIN=512

prior to an xfstests run, and these test binaries
should issue 512-aligned DIOs instead of whatever
XFS_IOC_DIOINFO says (i.e. instead of maybe 4k).

(This is in preparation for allowing 512 IOs on
"advanced format" 512/4k disks, when xfs has an
internal 4k sector size).

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>