generic: test extending sub-block AIO writes for races

generic: test extending sub-block AIO writes for races

This tests Brian Foster's fix for xfs:

   xfs: always drain dio before extending aio write submission

It launches four adjacent 1k IOs past EOF, then reads back
to see if we have 4k worth of the data we wrote, or something else -
possibly zeros from sub-block zeroing and eof racing.

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